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‎Interview/Company/facebook/algorithm_prepare.md‎

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4. [Best time to buy and sell stocks II] Medium
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5. [Best Time to Buy and Sell Stock III] Medium
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6. [Best time to buy and sell stock with transcation fee](#Best-time-to-buy-and-sell-stock-with-transcation-fee) Medium
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7. [Diagnose Tranverse](#Diagnose-Tranverse) Medium
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8. [Insert Delete GetRandom O(1)](#Insert-Delete-GetRandom-O(1)) Medium
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9. [Product of Array Except Self](#product-of-array-except-self) Medium
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10. [Kth Smallest Element in a Sorted Matrix](#Kth-Smallest-Element-in-a-Sorted-Matrix) Medium
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11. [Rotate Image](#Rotate-Image) Medium
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12. [Maximum Swap](#Maximum-Swap) Medium
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13. [Intersection of Two Arrays](#Intersection-of-Two-Arrays) Easy
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14. [Intersection of Two Arrays II](#Intersection-of-Two-Arrays-II) Easy
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15. [Sparse Matrix Multiplication](#Sparse-Matrix-Multiplication) Medium
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16. [Rectangle Overlap](#Rectangle-Overlap) Easy
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17. [Minimum Window Substring](#Minimum-Window-Substring) Hard
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18. [Valid Number](#Valid-Number) Hard
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19. [Candy](#Candy) Hard
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20. [Subarray Sum Equals K](#Subarray-Sum-Equals-K) Medium
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7. [Insert Delete GetRandom O(1)](#Insert-Delete-GetRandom-O(1)) Medium
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8. [Product of Array Except Self](#product-of-array-except-self) Medium
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9. [Maximum Swap](#Maximum-Swap) Medium
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10. [Intersection of Two Arrays](#Intersection-of-Two-Arrays) Easy
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11. [Intersection of Two Arrays II](#Intersection-of-Two-Arrays-II) Easy
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12. [Sparse Matrix Multiplication](#Sparse-Matrix-Multiplication) Medium
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13. [Rectangle Overlap](#Rectangle-Overlap) Easy
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14. [Minimum Window Substring](#Minimum-Window-Substring) Hard
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15. [Valid Number](#Valid-Number) Hard
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16. [Candy](#Candy) Hard
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17. [Subarray Sum Equals K](#Subarray-Sum-Equals-K) Medium
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18. [Find the Kth largest item](#Find-the-Kth-largest-item) Medium
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19. [Count Duplicates](#Count-Duplicates) Medium
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20. [3 Sum](#3-Ssum)
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## Matrix
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1. [Diagnose Tranverse](#Diagnose-Tranverse) Medium
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2. [Kth Smallest Element in a Sorted Matrix](#Kth-Smallest-Element-in-a-Sorted-Matrix) Medium
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3. [Rotate Image](#Rotate-Image) Medium
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## Binary Search
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1. [Search a 2D Matrix](#Search-a-2D-Matrix) Easy
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10. [Rotate List](#Rotate-List) Medium
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11. [Plus One Linked List](#Plus-One-Linked-List) Medium
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12. [Intersection of Two Linked Lists](#Intersection-of-Two-Linked-Lists) Medium
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13. [Remove Duplicates from Sorted List II](#Remove-Duplicates-from-Sorted-List-II) Medium
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14. [Remove Nth Node From End of List](#Remove-Nth node-from-the-end-of-list) Medium
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## String
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1. [Valid Anagram](#Valid-Anagram) Easy
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}
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return result;
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}
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```
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### Find the Kth largest item
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***Big O:*** O(n) speed, O(1) space
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```
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Tips:
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Quick Select
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```
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```c++
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class Solution {
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public:
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int findKthLargest(vector<int>& nums, int k) {
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//partition rule: >=pivot pivot <=pivot
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int left=0,right=nums.size()-1,idx=0;
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while(1){
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idx = partition(nums,left,right);
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if(idx==k-1) break;
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else if(idx < k-1) left=idx+1;
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else right= idx-1;
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}
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return nums[idx];
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}
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int partition(vector<int>& nums,int left,int right){//hoare partition
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int pivot = nums[left], l=left+1, r = right;
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while(l<=r){
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if(nums[l]<pivot && nums[r]>pivot) swap(nums[l++],nums[r--]);
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if(nums[l]>=pivot) ++l;
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if(nums[r]<=pivot) --r;
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}
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swap(nums[left], nums[r]);
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return r;
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}
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};
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```
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### Count Duplicates
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***Big O:*** O(n) speed, O(n) space
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```
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Tips:
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Hashmap
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```
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```c++
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class Solution {
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public:
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vector<int> countduplicates(vector<int> &nums) {
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// write your code here
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unordered_map<int, bool> dup;
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vector<int> ans;
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for (auto n : nums) {
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if ((dup.find(n) != dup.end()) && !dup[n]) {
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ans.push_back(n);
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dup[n] = true;
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}
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else if (dup.find(n) == dup.end())
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dup[n] = false;
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}
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return ans;
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}
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};
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```
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### 3 Sum
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***Big O:*** O(n^2) speed, O(1) space
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```
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Tips:
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Sort + 2 Sum II (two pointer).
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Be careful with duplicate items.
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```
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```c++
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class Solution {
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public:
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vector<vector<int>> threeSum(vector<int>& nums) {
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sort(begin(nums), end(nums));
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vector<vector<int>> res;
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for (int i = 0; i < nums.size() && nums[i] <= 0; ++i)
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if (i == 0 || nums[i - 1] != nums[i]) {
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twoSumII(nums, i, res);
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}
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return res;
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}
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void twoSumII(vector<int>& nums, int i, vector<vector<int>> &res) {
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int lo = i + 1, hi = nums.size() - 1;
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while (lo < hi) {
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int sum = nums[i] + nums[lo] + nums[hi];
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if (sum < 0) {
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++lo;
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} else if (sum > 0) {
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--hi;
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} else {
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res.push_back({ nums[i], nums[lo++], nums[hi--] });
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while (lo < hi && nums[lo] == nums[lo - 1])
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++lo;
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}
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}
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}
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};
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```
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### Remove Duplicates from Sorted List II
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***Big O:*** O(n) speed, O(1) space
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```
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Tips:
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Two pointers pre and cur.
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```
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```c++
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class Solution {
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public:
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ListNode* deleteDuplicates(ListNode* head) {
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ListNode sentinal, *pre, *cur, *nxt;
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sentinal.next = head;
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sentinal.val = INT_MIN;
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pre = &sentinal;
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cur = pre;
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while (cur && cur->next) {
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while (cur->next && cur->val == cur->next->val) {
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cur = cur->next;
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}
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cur = cur->next;
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pre->next = cur;
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if (cur && cur->next && cur->val != cur->next->val)
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pre = pre->next;
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}
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return sentinal.next;
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}
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};
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```
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### Remove Nth node from the end of list
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***Big O:*** O(L) speed, O(1) space
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```
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Tips:
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One pass algorithm:
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The first pointer advances the list by n+1n+1 steps from the beginning, while the second pointer starts from the beginning of the list. Now, both pointers are exactly separated by nn nodes apart. We maintain this constant gap by advancing both pointers together until the first pointer arrives past the last node. The second pointer will be pointing at the nnth node counting from the last. We relink the next pointer of the node referenced by the second pointer to point to the node's next next node.
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```
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```c++
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class Solution {
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public:
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ListNode* removeNthFromEnd(ListNode* head, int n) {
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if (head == nullptr || (head->next == nullptr && n > 0))
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return nullptr;
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ListNode sentinal, *first, *second;
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sentinal.next = head;
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first = &sentinal;
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second = &sentinal;
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while(n--) {
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if (second == nullptr)
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return nullptr;
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second = second->next;
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}
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while(second->next) {
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first = first->next;
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second = second->next;
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}
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first->next = first->next->next;
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return sentinal.next;
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}
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};
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```

‎Interview/SystemDesign/examples/airControlSystem.md‎

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I am not sure about above but I'd probably work from top to down and try to organize as much info as I can however time is limit in basically refining a design.
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## Players
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1. airplane in the sky
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2. air traffic control unit on the ground
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### Airplane -
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It has many components, but important one here are two
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- pilot - one who is displacing the plane or changing its position
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- air traffic control unit - the one that communicates with air traffic control system on the ground
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***Pilot -***
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It has couple of actions
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- register - sends initial message to new air traffic control on the ground
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- deregister - send final message to old air traffic control on the ground
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- steer - move plane (but not relevant to this i suppose)
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***Air Traffic Control Communication -***
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- updates its location - this happens either in loop continuously
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- send message to air traffic control unit on ground this happens again periodically in loop
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- receive messages from air traffic control unit and send those back to pilot - happens periodically in loop (messages are either simple pings or some message)
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### On the ground -
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***Controller***
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- subscribes for registration / deregistration / urgent messages on global queue
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- receive registration, send registration ack
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- receive de-registration, send re-registration ack
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- In response to registration spawn plane specific thread handling object
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( one can discuss thread pool, database/data structure to maintain location and other info of plane)
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***Worker:***
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- subscribes to messages on plane's queue
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- subscribes to messages on a global queue
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- update location database in response to periodic location update
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- notifies registration message and exit
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***Communication:***
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- A backend of radio. Implements message passing
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- Receives and collects all messages and distributes them
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- Messages are either registration, deregistration, urgent and location update
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- Messages are put in different queues , each worker thread create its own queue when it registers for messages and subscribes for it.
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***Control Unit on Plane***
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- subscribes for urgent messages from controller
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- periodically sends location update
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- Urgent messages directed to Pilot class
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- unsubscribe from controller
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***Pilot***
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- Sends registration, in response to ack, trigger Control Unit on Plane subscribe to new Controller
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- Sends deregistration, in response to ack, trigger Control Unit on Plane unsubscribe from old Controller
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- One can discuss message queue, subscription mechanism of IPC etc, DB, interfaces to GUI (if that counts)

‎README.md‎

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6. System Design Examples with Embedded Flavor
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1. Design a Cache
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2. Design a Vending Machine
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3. Design a Traffic Control System
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3. [Design a Traffic Control System](Interview/SystemDesign/examples/airControlSystem.md)
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4. Design a ATM Machine
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5. Design a Elevator System
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6. [Design a Parking Lot System](https://github.com/donnemartin/system-design-primer/blob/master/solutions/object_oriented_design/parking_lot/parking_lot.ipynb)
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7. Design a File System
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8. [Design a Garbage Collector](http://journal.stuffwithstuff.com/2013/12/08/babys-first-garbage-collector/)
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9. [Design a call center](https://github.com/donnemartin/system-design-primer/blob/master/solutions/object_oriented_design/call_center/call_center.ipynb)
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6. Design a Air Traffic Controller
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7. Design a Amazon Locker System
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8. Design a service for Alexa devices to report battery status
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9. [Design a Parking Lot System](https://github.com/donnemartin/system-design-primer/blob/master/solutions/object_oriented_design/parking_lot/parking_lot.ipynb)
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10. Design a File System
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11. [Design a Garbage Collector](http://journal.stuffwithstuff.com/2013/12/08/babys-first-garbage-collector/)
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12. [Design a call center](https://github.com/donnemartin/system-design-primer/blob/master/solutions/object_oriented_design/call_center/call_center.ipynb)
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7. [Embedded System Design Pattern Catalogue](https://embeddedartistry.com/fieldatlas/design-pattern-catalogue/)
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8. Embedded System Design Topics
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1. Small memory management

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