1. 两数之和

class Solution {
public:vector<int> twoSum(vector<int>& nums, int target) {vector<int> ans;map<int, int> mp;for (int i = 0; i < nums.size(); i++) {if (mp.count(target - nums[i])) {ans.push_back(mp[target - nums[i]]);ans.push_back(i);return ans;}mp[nums[i]] = i;}return ans;}
};

模拟

49. 字母异位词分组

class Solution {
public:vector<vector<string>> groupAnagrams(vector<string>& strs) {vector<vector<string>> res;if(strs.empty()){return res;}unordered_map<string,vector<string>> mp;for(auto str:strs){string key=str;sort(key.begin(),key.end());mp[key].emplace_back(str);}for(auto it=mp.begin();it!=mp.end();it++){res.emplace_back(it->second);}return res;}
};

模拟

128. 最长连续序列

class Solution {
public:int longestConsecutive(vector<int>& nums) {unordered_map<int, int> ump;for (auto x: nums) {ump[x]++;}int ans = 0;for (auto it = ump.begin(); it != ump.end(); it++) {int x = it->first;if (ump.count(x - 1)) continue;int t = x;int s = 0;while (ump.count(t)) {s++;t++;}ans = max(ans, s);}return ans;}
};

哈希，贪心

每次遇到只需要从一个连续序列的最开始的数查找即可