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

4sum.cpp

#include <string>
#include <vector>
#include <algorithm>

using namespace std;

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

        sort(nums.begin(), nums.end());
        vector<vector<int>> output;
        
        for (size_t i = 0; i < nums.size() - 3; i++)
        {
            if (i != 0 && nums[i] == nums[i - 1]) continue;
            if (target < (nums[i] + nums[i + 1] + nums[i + 2] + nums[i + 3])) break;
            if (target >(nums[i] + nums[nums.size() - 1] + nums[nums.size() - 2] + nums[nums.size() - 3])) continue;

            for (size_t j = i + 1; j < nums.size() - 2; j++)
            {
                if (j != i + 1 && nums[j] == nums[j - 1]) continue;
                if (target < (nums[i] + nums[j] + nums[j + 1] + nums[j + 2])) break;
                if (target >(nums[i] + nums[j] + nums[nums.size() - 1] + nums[nums.size() - 2])) continue;

                size_t front = j + 1;
                size_t back = nums.size() - 1;
                int sum = target - nums[i] - nums[j];
                while (front < back)
                {
                    if (nums[front] + nums[back] == sum)
                    {
                        output.push_back({ nums[i], nums[j], nums[front], nums[back] });
                        while ((front < back) && nums[front] == nums[front + 1]) front++;
                        while ((back < front) && nums[front] == nums[back - 1]) back--;
                        front++; back--;
                    }
                    else if (nums[front] + nums[back] < sum) front++;
                    else back--;
                }
                while ((j < nums.size() - 2) && nums[j] == nums[j + 1]) j++;
            }

            while ((i < nums.size() - 3) && nums[i] == nums[i + 1]) i++;
        }
        
        return output;
    }
};

int main()
{
    Solution s;
    auto o = s.fourSum({ -1, -5, -5, -3, 2, 5, 0, 4 }, -7);

    return 0;
}

first version:
Runtime: 44 ms
Memory: 9.2 MB

second version:
Runtime: 8 ms, faster than 100.00% of C++ online submissions for 4Sum.
Memory Usage: 9.4 MB, less than 100.00% of C++ online submissions for 4Sum.