18. 4Sum.cpp

class Solution {
public:
    vector<vector<int>> fourSum(vector<int>& nums, int target) {
            vector<vector<int>> result;
    int n = nums.size();
    if(n < 4) return result;

    sort(nums.begin(), nums.end());

    for(int i = 0; i < n - 3; i++) {
        if(i > 0 && nums[i] == nums[i - 1]) continue; // skip duplicates for i

        for(int j = i + 1; j < n - 2; j++) {
            if(j > i + 1 && nums[j] == nums[j - 1]) continue; // skip duplicates for j

            int left = j + 1;
            int right = n - 1;

            while(left < right) {
                long long sum = (long long)nums[i] + nums[j] + nums[left] + nums[right];
                
                if(sum == target) {
                    result.push_back({nums[i], nums[j], nums[left], nums[right]});

                    // skip duplicates for left
                    while(left < right && nums[left] == nums[left + 1]) left++;
                    // skip duplicates for right
                    while(left < right && nums[right] == nums[right - 1]) right--;

                    left++;
                    right--;
                }
                else if(sum < target) left++;
                else right--;
            }
        }
    }
    return result;
    }
};