兩數之和

1. 兩數之和 - 力扣（LeetCode）

直接暴力

class Solution {public int[] twoSum(int[] nums, int target) {for(int i=0;i<nums.length;i++){for(int j=i+1;j<nums.length;j++){long ans = nums[i]+nums[j];if(ans>target)continue;if(ans==target){return new int[]{i,j};}}}return null;}
}

哈希表優化

class Solution {public int[] twoSum(int[] nums, int target) {HashMap<Integer,Integer>hm=new HashMap<>();for(int i=0;i<nums.length;i++){hm.put(nums[i],i);}for(int i=0;i<nums.length;i++){if(hm.containsKey(target-nums[i])){int j=hm.get(target-nums[i]);if(i!=j){return new int[]{i,j};}}}return null;}
}

字母異位詞分組

49. 字母異位詞分組 - 力扣（LeetCode）

class Solution {public List<List<String>> groupAnagrams(String[] strs) {Map<String, List<String>> map = new HashMap<String, List<String>>();for (String str : strs) {char[] array = str.toCharArray();Arrays.sort(array);String key = new String(array);List<String> list = map.getOrDefault(key, new ArrayList<String>());list.add(str);map.put(key, list);}return new ArrayList<List<String>>(map.values());}
}

最長連續序列

128. 最長連續序列 - 力扣（LeetCode）

class Solution {public int longestConsecutive(int[] nums) {Arrays.sort(nums);int max=0;HashMap<Integer,Integer>hm=new HashMap<>();for(int i=0;i<nums.length;i++){int num=nums[i];if(hm.containsKey(num))continue;else{if(hm.containsKey(num-1)){hm.put(num,hm.get(num-1)+1);}else{hm.put(num,1);}}max=Math.max(max,hm.get(num));}return max;}
}