4Sum.cpp

/*
Given an array nums of n integers, return an array of all the unique quadruplets [nums[a], nums[b], nums[c], nums[d]] such that:

0 <= a, b, c, d < n
a, b, c, and d are distinct.
nums[a] + nums[b] + nums[c] + nums[d] == target
You may return the answer in any order.

 

Example 1:

Input: nums = [1,0,-1,0,-2,2], target = 0
Output: [[-2,-1,1,2],[-2,0,0,2],[-1,0,0,1]]
Example 2:

Input: nums = [2,2,2,2,2], target = 8
Output: [[2,2,2,2]]
 

Constraints:

1 <= nums.length <= 200
-109 <= nums[i] <= 109
-109 <= target <= 109
*/
class Solution {
public:
    vector<vector<int>> fourSum(vector<int>& nums, int target) {
        int n = nums.size();
        vector<vector<int>> result;
        sort(nums.begin(),nums.end());
        for(int i=0;i<n;i++)
        {
            for(int j=i+1;j<n;j++)
            {
                int left = j+1, right = n-1;
                while(left < right)
                {
                    int sum = nums[i] + nums[j] + nums[left] + nums[right];
                    if(sum == target)
                    {
                        result.push_back({nums[i],nums[j],nums[left],nums[right]});
                        while(left < right-2 && nums[left+1] == nums[left])
                            left++;
                        while(left < right-2 && nums[right-1] == nums[right])
                            right--;
                        left++;
                        right--;
                    }
                    else if(sum < target)
                        left++;
                    else
                        right--;
                }
                while(j < n-1 && nums[j+1] == nums[j])
                    j++;
            }
            while(i < n-1 && nums[i+1] == nums[i])
                i++;
        }
        return result;
    }
};