1using System;
2using System.Collections.Generic;
3
4class FourSumSolution {
5    public IList<IList<int>> FourSum(int[] nums, int target) {
6        Array.Sort(nums);
7        List<IList<int>> result = new List<IList<int>>();
8        int n = nums.Length;
9        for (int i = 0; i < n - 3; i++) {
10            if (i > 0 && nums[i] == nums[i - 1]) continue;
11            for (int j = i + 1; j < n - 2; j++) {
12                if (j > i + 1 && nums[j] == nums[j - 1]) continue;
13                int k = j + 1, l = n - 1;
14                while (k < l) {
15                    int sum = nums[i] + nums[j] + nums[k] + nums[l];
16                    if (sum == target) {
17                        result.Add(new List<int>{nums[i], nums[j], nums[k], nums[l]});
18                        while (k < l && nums[k] == nums[k + 1]) k++;
19                        while (k < l && nums[l] == nums[l - 1]) l--;
20                        k++; l--;
21                    } else if (sum < target) {
22                        k++;
23                    } else {
24                        l--;
25                    }
26                }
27            }
28        }
29        return result;
30    }
31
32    static void Main() {
33        var solver = new FourSumSolution();
34        int[] nums = {1, 0, -1, 0, -2, 2};
35        int target = 0;
36        var result = solver.FourSum(nums, target);
37        foreach (var r in result) {
38            Console.WriteLine("[{0}]", string.Join(", ", r));
39        }
40    }
41}

1#include <unordered_map>
#include <vector>
using namespace std;

vector<vector<int>> fourSum(vector<int>& nums, int target) {
    sort(nums.begin(), nums.end());
    vector<vector<int>> result;
    int n = nums.size();
    for (int i = 0; i < n - 3; i++) {
        if (i > 0 && nums[i] == nums[i - 1]) continue;
        for (int j = i + 1; j < n - 2; j++) {
            if (j > i + 1 && nums[j] == nums[j - 1]) continue;
            unordered_map<int, int> map;
            for (int k = j + 1; k < n; k++) {
                if (map.count(nums[k])) {
                    result.push_back({nums[i], nums[j], nums[map[nums[k]]], nums[k]});
                    while (k + 1 < n && nums[k] == nums[k + 1]) k++;
                } else {
                    map[target - nums[i] - nums[j] - nums[k]] = k;
                }
            }
        }
    }
    return result;
}

int main() {
    vector<int> nums = {1, 0, -1, 0, -2, 2};
    int target = 0;
    vector<vector<int>> result = fourSum(nums, target);
    for (auto &quad : result) {
        cout << "[";
        for (int i = 0; i < quad.size(); i++) {
            cout << quad[i] << (i == quad.size() - 1 ? "" : ", ");
        }
        cout << "]\n";
    }
    return 0;
}