两数之和
class Solution {
public:vector<int> twoSum(vector<int>& nums, int target) {unordered_map<int,int> map;for(int i=0;i<nums.size();i++){if(map.count(target-nums[i])!=0){return vector<int>{i,map[target-nums[i]]};}else{map[nums[i]]=i;}}return vector<int>{0,0};}
};

四数相加II
class Solution {
public:int fourSumCount(vector<int>& nums1, vector<int>& nums2, vector<int>& nums3, vector<int>& nums4) {unordered_map<int,int> map;for(int a:nums1){for(int b:nums2){map[a+b]++;}}int result=0;for(int c:nums3){for(int d:nums4){if(map.count(0-(c+d))!=0){result+=map[0-(c+d)];}}}return result;}
};