<p>Given an array <code>nums</code> of <code>n</code> integers, return <em>an array of all the <strong>unique</strong> quadruplets</em> <code>[nums[a], nums[b], nums[c], nums[d]]</code> such that:</p>

<ul>
	<li><code>0 &lt;= a, b, c, d&nbsp;&lt; n</code></li>
	<li><code>a</code>, <code>b</code>, <code>c</code>, and <code>d</code> are <strong>distinct</strong>.</li>
	<li><code>nums[a] + nums[b] + nums[c] + nums[d] == target</code></li>
</ul>

<p>You may return the answer in <strong>any order</strong>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> nums = [1,0,-1,0,-2,2], target = 0
<strong>Output:</strong> [[-2,-1,1,2],[-2,0,0,2],[-1,0,0,1]]
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> nums = [2,2,2,2,2], target = 8
<strong>Output:</strong> [[2,2,2,2]]
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= nums.length &lt;= 200</code></li>
	<li><code>-10<sup>9</sup> &lt;= nums[i] &lt;= 10<sup>9</sup></code></li>
	<li><code>-10<sup>9</sup> &lt;= target &lt;= 10<sup>9</sup></code></li>
</ul>

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

class Solution {
public:
    vector<vector<int>> twoSum(vector<int>& nums, long long target, int start) {
        unordered_map<int, int> mep;
        vector<vector<int>> ans;
        for (int i = start; i < nums.size(); i++) {
            long long complement = target - nums[i];
            if (mep.find(complement) != mep.end() && mep[complement] != i) {
                ans.push_back({(int)complement, nums[i]});
                while (i + 1 < nums.size() && nums[i] == nums[i + 1]) i++;
            }
            mep[nums[i]] = i;
        }
        return ans;
    }

    vector<vector<int>> threeSum(vector<int>& nums, long long target, int start) {
        vector<vector<int>> ans;
        int n = nums.size();
        for (int i = start; i < n - 2; i++) {
            if (i > start && nums[i] == nums[i - 1]) continue;
            long long newTarget = target - nums[i];
            vector<vector<int>> output = twoSum(nums, newTarget, i + 1);
            for (auto &pair : output) {
                pair.push_back(nums[i]);
                ans.push_back(pair);
            }
        }
        return ans;
    }

    vector<vector<int>> fourSum(vector<int>& nums, long long target) {
        vector<vector<int>> ans;
        sort(nums.begin(), nums.end());
        int n = nums.size();
        for (int i = 0; i < n - 3; i++) {
            if (i > 0 && nums[i] == nums[i - 1]) continue;
            long long smallestSum = (long long)nums[i] + nums[i + 1] + nums[i + 2] + nums[i + 3];
            long long largestSum = (long long)nums[i] + nums[n - 1] + nums[n - 2] + nums[n - 3];
            if (smallestSum > target) break;
            if (largestSum < target) continue;

            long long newTarget = target - nums[i];
            vector<vector<int>> output = threeSum(nums, newTarget, i + 1);
            for (auto &tuple : output) {
                tuple.push_back(nums[i]);
                ans.push_back(tuple);
            }
        }
        sort(ans.begin(), ans.end());
        ans.erase(unique(ans.begin(), ans.end()), ans.end());
        return ans;
    }
};

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