1. 两数之和

 

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