力扣hot100——哈希

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