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ys-han00ys-han00
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Merge pull request #2025 from ys-han00/main
[ys-han00] WEEK 02 solutions
2 parents c59a6ba + 0a07a70 commit cd317ff

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‎3sum/ys-han00.cpp‎

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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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// set<vector<int>> ans;
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// map<int, int> counter; // {num : cnt}
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// vector<int> targets;
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// int target, left, right;
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// for(int i = 0; i < nums.size(); i++)
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// counter[nums[i]]++;
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// for(auto iter : counter)
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// targets.push_back(iter.first);
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// for(int i = 0; i < targets.size(); i++) {
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// target = -1 * targets[i];
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// vector<int> new_nums;
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// counter[targets[i]]--;
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// for(auto iter : counter) {
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// for(int j = 0; j < iter.second; j++)
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// new_nums.push_back(iter.first);
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// }
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// left = 0; right = new_nums.size() - 1;
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// while(left < right) {
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// int sum = new_nums[left] + new_nums[right];
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// if(sum == target && targets[i] <= new_nums[left] && new_nums[left] <= new_nums[right])
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// ans.insert(vector<int>({targets[i], new_nums[left], new_nums[right]}));
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// if(sum < target)
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// left++;
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// else
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// right--;
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// }
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// counter[targets[i]]++;
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// }
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// return vector<vector<int>> (ans.begin(), ans.end());
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// }
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// };
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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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vector<vector<int>> ans;
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int left, right, sum;
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sort(nums.begin(), nums.end());
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for(int i = 0; i < nums.size(); i++) {
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if(i > 0 && nums[i] == nums[i - 1])
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continue;
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left = i + 1; right = nums.size() - 1;
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while(left < right) {
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sum = nums[i] + nums[left] + nums[right];
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if(sum == 0) {
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ans.push_back(vector<int> ({nums[i], nums[left], nums[right]}));
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left++; right--;
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while(left < right && nums[left] == nums[left - 1])
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left++;
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while(left < right && nums[right] == nums[right + 1])
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right--;
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}
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else if(sum < 0)
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left++;
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else if(sum > 0)
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right--;
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}
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}
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return ans;
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}
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};
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‎climbing-stairs/ys-han00.cpp‎

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class Solution {
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public:
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int climbStairs(int n) {
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vector<int> fibo(46, 0);
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fibo[1] = 1;
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fibo[2] = 2;
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if(n <= 2)
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return fibo[n];
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for(int i = 3; i <= n; i++)
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fibo[i] = fibo[i - 1] + fibo[i - 2];
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return fibo[n];
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}
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};
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‎product-of-array-except-self/ys-han00.cpp‎

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// class Solution {
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// public:
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// vector<int> productExceptSelf(vector<int>& nums) {
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// int mul = 1, zero_cnt = 0;
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// for(int i = 0; i < nums.size(); i++) {
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// if(nums[i] != 0)
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// mul *= nums[i];
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// else
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// zero_cnt++;
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// }
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// vector<int> ans = vector<int>(nums.size(), 0);
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// if(zero_cnt == 1) {
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// for(int i = 0; i < nums.size(); i++) {
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// if(nums[i] == 0) {
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// ans[i] = mul;
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// break;
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// }
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// }
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// } else if(zero_cnt == 0) {
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// for(int i = 0; i < nums.size(); i++)
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// ans[i] = mul / nums[i];
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// }
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// return ans;
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// }
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// };
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// class Solution {
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// public:
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// vector<int> productExceptSelf(vector<int>& nums) {
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// int mul = 1, zero_cnt = 0;
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// vector<int> prefix(nums.size(), 0), postfix(nums.size(), 0);
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// prefix[0] = 1;
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// for(int i = 1; i < nums.size(); i++)
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// prefix[i] = nums[i - 1] * prefix[i - 1];
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// postfix[nums.size() - 1] = 1;
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// for(int i = nums.size() - 2; i >= 0; i--)
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// postfix[i] = nums[i + 1] * postfix[i + 1];
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// vector<int> ans = vector<int>(nums.size(), 0);
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// for(int i = 0; i < nums.size(); i++)
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// ans[i] = postfix[i] * prefix[i];
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// return ans;
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// }
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// };
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class Solution {
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public:
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vector<int> productExceptSelf(vector<int>& nums) {
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vector<int> ans(nums.size(), 1);
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int before = 1, after = 1;
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for(int i = 0; i < nums.size() - 1; i++) {
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before *= nums[i];
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ans[i + 1] *= before;
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}
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for(int i = nums.size() - 1; i > 0; i--) {
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after *= nums[i];
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ans[i - 1] *= after;
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}
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return ans;
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}
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};
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‎valid-anagram/ys-han00.cpp‎

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// class Solution {
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// public:
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// bool isAnagram(string s, string t) {
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// map<char, int> counter;
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// for(int i = 0; i < s.size(); i++)
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// counter[s[i] - 'a']++;
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// for(int i = 0; i < t.size(); i++)
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// counter[t[i] - 'a']--;
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// for(auto const& iter : counter)
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// if(iter.second != 0)
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// return false;
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// return true;
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// }
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// };
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class Solution {
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public:
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bool isAnagram(string s, string t) {
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vector<int> counter(26, 0);
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for(int i = 0; i < s.size(); i++)
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counter[s[i] - 'a']++;
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for(int i = 0; i < t.size(); i++)
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counter[t[i] - 'a']--;
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for(int i = 0; i < 26; i++)
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if(counter[i] != 0)
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return false;
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return true;
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}
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};
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‎validate-binary-search-tree/ys-han00.cpp‎

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/**
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* Definition for a binary tree node.
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* struct TreeNode {
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* int val;
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* TreeNode *left;
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* TreeNode *right;
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* TreeNode() : val(0), left(nullptr), right(nullptr) {}
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* TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
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* TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
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* };
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*/
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// class Solution {
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// public:
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// bool isValidBST(TreeNode* root) {
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// queue<pair<TreeNode*, vector<pair<int, int>>>> que;
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// int val, dir, num;
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// que.push({root, vector<pair<int, int>> ()});
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// while(!que.empty()) {
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// TreeNode* node = que.front().first;
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// vector<pair<int, int>> path = que.front().second;
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// que.pop();
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// val = node->val;
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// for(int i = 0; i < path.size(); i++) {
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// num = path[i].first;
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// dir = path[i].second;
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// if(dir == 0 && num <= val)
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// return false;
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// if(dir == 1 && val <= num)
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// return false;
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// }
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// if(!node->left && !node->right)
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// continue;
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// if(node->left) {
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// path.push_back({val, 0});
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// que.push({node->left, path});
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// path.pop_back();
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// }
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// if(node->right) {
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// path.push_back({val, 1});
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// que.push({node->right, path});
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// path.pop_back();
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// }
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// }
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// return true;
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// }
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// };
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class Solution {
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public:
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void inorder(TreeNode* root, vector<int>& ordered) {
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if(root==NULL)
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return;
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inorder(root->left, ordered);
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ordered.push_back(root->val);
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inorder(root->right, ordered);
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}
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bool isValidBST(TreeNode* root) {
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vector<int> ordered;
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inorder(root, ordered);
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for(int i = 1; i < ordered.size(); i++)
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if(ordered[i - 1] >= ordered[i])
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return false;
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return true;
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}
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};
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