foursum.c++

#include <iostream>
#include <vector>
#include <algorithm>

std::vector<std::vector<int>> fourSum(std::vector<int>& nums, int target) {
    std::vector<std::vector<int>> result;
    int n = nums.size();
    
    if (n < 4) {
        return result;  // There are not enough elements for a quadruplet.
    }
    
    std::sort(nums.begin(), nums.end());  // Sort the array.

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

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

            int left = j + 1;
            int 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 && nums[left] == nums[left + 1]) left++;  // Skip duplicates.
                    while (left < right && nums[right] == nums[right - 1]) right--;  // Skip duplicates.
                    
                    left++;
                    right--;
                } else if (sum < target) {
                    left++;
                } else {
                    right--;
                }
            }
        }
    }

    return result;
}

int main() {
    std::vector<int> nums = {1, 0, -1, 0, -2, 2};
    int target = 0;
    
    std::vector<std::vector<int>> result = fourSum(nums, target);
    
    for (const std::vector<int>& quad : result) {
        for (int num : quad) {
            std::cout << num << " ";
        }
        std::cout << std::endl;
    }
    
    return 0;
}