<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>& numsIn, int target) {
        if (numsIn.size() < 4) return {};

        vector<int> nums;
        std::sort(numsIn.begin(), numsIn.end());
        int lastNum = 0;
        int count = -1;
        for (int & num : numsIn) {
            if (lastNum == num) {
                count += 1;
                if (count < 4) {
                    nums.push_back(num);
                }
            } else {
                lastNum = num;
                count = 0;
                nums.push_back(num);
            }
        }
        
        /*for (int & num : nums) {
            cout << "num: " << num << endl;
        }*/

        vector<vector<int>> sols, uniquesols;
        
        for (int i = 0; i < nums.size() - 3; ++i) {
            vector<int> subset = nums;
            subset.erase(subset.begin(), subset.begin() + i + 1);
            for (auto subsol : threeSum(subset, int64_t(target) - nums[i])) {
                subsol.push_back(nums[i]);
                std::sort(subsol.begin(), subsol.end());
                sols.push_back(subsol);
            }
        }

        std::sort(sols.begin(), sols.end());

        std::unique_copy(sols.begin(), sols.end(), std::back_inserter(uniquesols));

        return uniquesols;
    }

    vector<vector<int>> threeSum(vector<int>& nums, int64_t target) {
        vector<vector<int>> sols;
        
        for (int i = 0; i < nums.size() - 2; ++i) {
            vector<int> subset = nums;
            subset.erase(subset.begin(), subset.begin() + i + 1);
            for (auto subsol : twoSum(subset, target - nums[i])) {
                subsol.push_back(nums[i]);
                sols.push_back(subsol);
            }
        }

        return sols;
    }

    vector<vector<int>> twoSum(vector<int>& numsIn, int64_t target) {
        vector<vector<int>> sols;
        vector<int64_t> nums;
        for (int num : numsIn) nums.push_back(num);

        std::sort(nums.begin(), nums.end());
        for (int64_t & num : nums) {
            num = num * 2 - target;
        }

        auto i = std::find_if(nums.begin(), nums.end(), [](int64_t& num) { return num >= 0; } );
        auto j = i + 1;
        if (i == nums.end()) return {};
        if (*i > 0) {
            --i;
            --j;
        }
        while (i >= nums.begin() && j < nums.end()) {
            int sum = *i + *j;
            if (sum == 0) {
                sols.push_back({int((*i + target) / 2), int((*j + target) / 2)});
                --i;
                continue;
            }
            if (sum > 0) {
                --i;
            } else {
                ++j;
            }
        }
        return sols;
    }
};
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