https://leetcode.com/problems/4sum/

#include<algorithm>
#include<vector>
using namespace std;
class Solution {
public:
    void _sum(vector<int>& nums,int l,vector<vector<int>> &ans,int target,vector<int> &x,int n) {
        int L=nums.size();
        if(n>L-l || n*nums[l] > target || n*nums.back()<target) return;
        if(n==2){
            
            int r=nums.size()-1;
            while(l<r){
                if(nums[l]+nums[r]>target) --r;
                else if(nums[l]+nums[r]<target) ++l;
                else{
                    x.push_back(nums[l]);
                    x.push_back(nums[r]);
                    ans.push_back(x);
                    x.pop_back();
                    x.pop_back();
                    ++l;
                    --r;
                    while(l<r && nums[l]==nums[l-1]) ++l;
                    while(l<r && nums[r]==nums[r+1]) --r;
                }
            }
        }
        else{
            
            for(int i=l;i<=L-n;++i){
                if(i!=l && nums[i]==nums[i-1]) continue;
                x.push_back(nums[i]);
                vector<vector<int>> tmp;
                _sum(nums,i+1,tmp,target-nums[i],x,n-1);
                for(auto &xx:tmp)
                    ans.push_back(xx);
                x.pop_back();
            }
        }
        return;

    }

    vector<vector<int>> fourSum(vector<int>& nums, int target) {
        /*
        和第15题共用以上函数
        */
        
        sort(nums.begin(),nums.end());
        vector<vector<int>> ans;
        vector<int> x;
        _sum(nums,0,ans,target,x,4);
        return ans;
    }
};