https://leetcode.com/problems/3sum-closest/

#include<algorithm>
using namespace std;
class Solution {
public:
    void _sum(vector<int>& nums,int l,int &ans,int target,int x,int n) {
        int L=nums.size();
        if(n==2){
            int r=nums.size()-1;
            while(l<r){
                if(nums[l]+nums[r]+x>target){
                    if(abs(ans-target) > abs(nums[l]+nums[r]+x-target)) ans=nums[l]+nums[r]+x;
                    --r;
                }
                else if(nums[l]+nums[r]+x<target){
                    if(abs(ans-target) > abs(nums[l]+nums[r]+x-target)) ans=nums[l]+nums[r]+x;
                    ++l;
                }
                else{
                    ans=target;
                    return ;
                }
            }
        }
        else{
            
            for(int i=l;i<=L-n;++i){
                if(i!=l && nums[i]==nums[i-1]) continue;
                x+=nums[i];
                _sum(nums,i+1,ans,target,x,n-1);
                if(ans==target) return;
                x-=nums[i];
            }
        }
        return;
    }
    int threeSumClosest(vector<int>& nums, int target) {
        /*
        对leetcode 15的函数略作修改即可
        */
        sort(nums.begin(),nums.end());
        int ans=nums[0]+nums[1]+nums[2],x=0;
         _sum(nums,0,ans,target,x,3);
        return ans;
    }
};