1. 两数之和(c++/哈希)

暴力for：

class Solution {
public:
    vector twoSum(vector& nums, int target) {
        for(int i = 0;i < nums.size();i++){
            for(int j = i + 1;j < nums.size();j++){
                if(nums[i]+nums[j]==target){
                    return{i,j};
                }
            }
        }
        return {};

    }
};

哈希：

class Solution {
public:
    vector twoSum(vector& nums, int target) {
        unordered_maphash_map;
        for(int i = 0;i < nums.size();i++){
            auto it = hash_map.find(target - nums[i]);
            if(it != hash_map.end()){
                return {it->second,i};
            }
            hash_map[nums[i]] = i;
        }
        return {};

    }
};