Given an array nums of n integers and an integer target, are there elements a, b, c, and d in nums such that a + b + c + d = target? Find all unique quadruplets in the array which gives the sum of target.

Note:

The solution set must not contain duplicate quadruplets.

Example:

Given array nums = [1, 0, -1, 0, -2, 2], and target = 0.

A solution set is:

[
	[-1, 0, 0, 1],
	[-2, -1, 1, 2],
	[-2, 0, 0, 2]
]


Solution in C++

class Solution {
public:
    vector<vector<int>> fourSum(vector<int>& nums, int target) {
        sort(nums.begin(), nums.end());
        
        map<int,int> mp;
        for(int i=0; i<nums.size(); i++) mp[nums[i]]=i+1;
        
        set<vector<int>> sRet; // storage will contain results
        
        for(int i=0; i<nums.size(); i++)
            for(int j=i+1; j<nums.size(); j++)
                for(int k=j+1; k<nums.size(); k++) {
                    int l = target - nums[i] - nums[j] - nums[k];
                    if (mp[l] && (mp[l]-1)>k) {
                        vector<int> tmp = {nums[i], nums[j], nums[k], l};                        
                        sRet.insert(tmp);
                    }
                }
        vector<vector<int>> ret;
        for(auto n:sRet) ret.push_back(n);
        return ret;
    }
};