[minji-go] Week 1

contains-duplicate/minji-go.java

@@ -1,3 +1,13 @@
+/*
+    Problem: https://leetcode.com/problems/contains-duplicate/
+    Description: return true if any value appears at least twice in the array
+    Concept: Array, Hash Table, Sorting
+    Time Complexity: O(n), Runtime: 10ms
+    Space Complexity: O(n), Memory: 58.6MB
+*/
+import java.util.HashSet;
+import java.util.Set;
+
 class Solution {
     public boolean containsDuplicate(int[] nums) {
         Set<Integer> count = new HashSet<>();

house-robber/minji-go.java

@@ -1,11 +1,19 @@
+/*
+    Problem: https://leetcode.com/problems/house-robber/
+    Description: the maximum amount of money you can rob if you cannot rob two adjacent houses
+    Concept: Array, Dynamic Programming
+    Time Complexity: O(n), Runtime: 0ms
+    Space Complexity: O(1), Memory: 41.42MB
+*/
 class Solution {
     public int rob(int[] nums) {
-        int[] sum = new int[nums.length+1];
-        sum[0] = nums[0];
-        if(nums.length>1) sum[1] = Math.max(nums[0], nums[1]);
+        int sum1 = nums[0];
+        int sum2 = nums.length>1? Math.max(nums[0], nums[1]) : nums[0];
         for(int i=2; i<nums.length; i++){
-            sum[i]=Math.max(nums[i]+sum[i-2],sum[i-1]);
+            int sum3=Math.max(nums[i]+sum1,sum2);
+            sum1=sum2;
+            sum2=sum3;
         }
-        return sum[nums.length-1];
+        return sum2;
     }
-}
+}

longest-consecutive-sequence/minji-go.java

@@ -1,3 +1,13 @@
+/*
+    Problem: https://leetcode.com/problems/longest-consecutive-sequence/
+    Description: return the length of the longest consecutive elements sequence
+    Concept: Array, Hash Table, Union Find
+    Time Complexity: O(n), Runtime: 1141ms
+    Space Complexity: O(n), Memory: 66.74MB
+*/
+import java.util.HashSet;
+import java.util.Set;
+
 class Solution {
     public int longestConsecutive(int[] nums) {
         Set<Integer> set = new HashSet<>();
@@ -7,7 +17,7 @@ public int longestConsecutive(int[] nums) {
 
         int answer = 0;
         for(int num: nums){
-            if(set.contains(num-1)){
+            if(set.contains(num-1)){ //contains(): O(1)
                 continue;
             }
             int length = 1;

top-k-frequent-elements/minji-go.java

@@ -1,17 +1,28 @@
+/*
+    Problem: https://leetcode.com/problems/top-k-frequent-elements/
+    Description: return the k most frequent elements
+    Concept: Array, Hash Table, Divide and Conquer, Sorting, Heap (Priority Queue), Bucket Sort, Counting, Quickselect
+    Time Complexity: O(n log k), Runtime: 15ms
+    Space Complexity: O(n), Memory: 48.64MB
+*/
+import java.util.*;
+
 class Solution {
     public int[] topKFrequent(int[] nums, int k) {
         Map<Integer, Integer> count = new HashMap<>();
         for(int num : nums){
             count.put(num, count.getOrDefault(num, 0)+1);
         }
-        List<Integer> tops = count.keySet().stream()
-                .sorted((i1, i2) -> count.get(i2)-count.get(i1))
-                .limit(k)
-                .toList();
+
+        PriorityQueue<Integer> pq = new PriorityQueue<>(Comparator.comparingInt(count::get));
+        for (int num : count.keySet()) {
+            pq.offer(num);
+            if (pq.size() > k) pq.poll();
+        }
 
         int[] answer = new int[k];
         for(int i=0; i<k; i++) {
-            answer[i] = tops.get(i);
+            answer[i] = pq.poll();
         }
         return answer;
     }

valid-palindrome/minji-go.java

@@ -1,7 +1,14 @@
+/*
+    Problem: https://leetcode.com/problems/valid-palindrome/
+    Description: return true if it is a palindrome, alphanumeric characters(letters and numbers) reads the same forward and backward
+    Concept: Two Pointers, String
+    Time Complexity: O(n), Runtime: 10ms
+    Space Complexity: O(n), Memory: 58.6MB
+*/
 class Solution {
     public boolean isPalindrome(String s) {
         String regex ="[^A-Za-z0-9]";
-        String palindrome = s.replaceAll(regex,"").toLowerCase();
+        String palindrome = s.replaceAll(regex,"").toLowerCase(); //replaceAll(), toLowerCase(): O(n)
 
         boolean answer = true;
         for(int i=0; i<palindrome.length()/2; i++){