Merge pull request DaleStudy#649 from minji-go/main

[minji-go] Week 1

Author: SamTheKorean

contains-duplicate/minji-go.java

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+/*
+    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<>();
+        boolean answer = false;
+        for(int num : nums){
+            if(count.contains(num)) {
+                answer = true;
+                break;
+            }
+            count.add(num);
+        }
+        return answer;
+    }
+}

house-robber/minji-go.java

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+/*
+    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 sum1 = nums[0];
+        int sum2 = nums.length>1? Math.max(nums[0], nums[1]) : nums[0];
+        for(int i=2; i<nums.length; i++){
+            int sum3=Math.max(nums[i]+sum1,sum2);
+            sum1=sum2;
+            sum2=sum3;
+        }
+        return sum2;
+    }
+}

longest-consecutive-sequence/minji-go.java

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+/*
+    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<>();
+        for(int num: nums) {
+            set.add(num);
+        }
+
+        int answer = 0;
+        for(int num: nums){
+            if(set.contains(num-1)){ //contains(): O(1)
+                continue;
+            }
+            int length = 1;
+            while (set.contains(num+length)){
+                length++;
+            }
+            answer = Math.max(answer, length);
+        }
+
+        return answer;
+    }
+}

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

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+/*
+    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);
+        }
+
+        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] = pq.poll();
+        }
+        return answer;
+    }
+}

valid-palindrome/minji-go.java

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+/*
+    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(); //replaceAll(), toLowerCase(): O(n)
+
+        boolean answer = true;
+        for(int i=0; i<palindrome.length()/2; i++){
+            if(palindrome.charAt(i) != palindrome.charAt(palindrome.length()-1-i)) {
+                answer = false;
+                break;
+            }
+        }
+        return answer;
+    }
+}