<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>

class Solution {
public:
    vector<vector<int>> fourSum(vector<int>& nums, int target) {
        int n = nums.size();
        set<vector<int>> st;  // To store unique quadruplets

        for(int i = 0; i < n; i++) {
            for(int j = i + 1; j < n; j++) {

                unordered_set<long long> hashset;  // Use unordered_set to track elements we've seen so far

                for(int k = j + 1; k < n; k++) {
                    // Cast sum to long long to avoid overflow
                    long long sum = (long long)nums[i] + (long long)nums[j] + (long long)nums[k];
                    long long toFind = (long long)target - sum;

                    // Check if toFind exists in the hashset
                    if(hashset.find(toFind) != hashset.end()) {
                        vector<int> temp = {nums[i], nums[j], nums[k], (int)toFind};
                        sort(temp.begin(), temp.end());
                        st.insert(temp);  // Insert sorted quadruplet to ensure uniqueness
                    }

                    // Add current element to the hashset
                    hashset.insert(nums[k]);
                }
            }
        }

        // Convert set to vector of vectors for result
        vector<vector<int>> ans(st.begin(), st.end());
        return ans;
    }
};

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