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

程式碼：

class Solution {
public:
    vector<vector<int>> fourSum(vector<int>& nums, int target) {
        vector<vector<int> > res;
        if(nums.size()==0) return res;
        sort(nums.begin(),nums.end());
        for(int i = 0;i<nums.size();i++){
            int sum1 = target - nums[i];
            for(int j = i+1;j<nums.size();j++){
                int sum2 = sum1 - nums[j];
                int start = j+1,end = nums.size()-1;
                while(start < end){
                    if(nums[start]+nums[end]<sum2){
                        start++;
                    }else if(nums[start]+nums[end]>sum2){
                        end --;
                    }else{
                        vector<int> temp = {nums[i],nums[j],nums[start],nums[end]};
                        res.push_back(temp);
                        while(start<end&&nums[start]==temp[2]) start++;
                        while(start<end&&nums[end]==temp[3]) end--;                        
                    }
                }
                 while(j+1<nums.size()&&nums[j+1]==nums[j]) j++;
            }
            while(i+1<nums.size()&&nums[i+1]==nums[i]) i++;
        }
        return res;
    }
};