<p>Given an integer array of even length <code>arr</code>, return <code>true</code><em> if it is possible to reorder </em><code>arr</code><em> such that </em><code>arr[2 * i + 1] = 2 * arr[2 * i]</code><em> for every </em><code>0 &lt;= i &lt; len(arr) / 2</code><em>, or </em><code>false</code><em> otherwise</em>.</p>

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

<pre>
<strong>Input:</strong> arr = [3,1,3,6]
<strong>Output:</strong> false
</pre>

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

<pre>
<strong>Input:</strong> arr = [2,1,2,6]
<strong>Output:</strong> false
</pre>

<p><strong class="example">Example 3:</strong></p>

<pre>
<strong>Input:</strong> arr = [4,-2,2,-4]
<strong>Output:</strong> true
<strong>Explanation:</strong> We can take two groups, [-2,-4] and [2,4] to form [-2,-4,2,4] or [2,4,-2,-4].
</pre>

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

<ul>
	<li><code>2 &lt;= arr.length &lt;= 3 * 10<sup>4</sup></code></li>
	<li><code>arr.length</code> is even.</li>
	<li><code>-10<sup>5</sup> &lt;= arr[i] &lt;= 10<sup>5</sup></code></li>
</ul>

class Solution {
public:
    bool containsDuplicates(vector<int>& arr, std::unordered_map<int, int>& element_counts) {
        // loop through arr
        for (unsigned int i = 0; i < arr.size(); i++) {
            // check if num has already been used
            int num = arr.at(i);
            if (!element_counts.contains(num)) {
                continue;
            }

            // decrement num in map
            element_counts.at(num)--;
            if (element_counts.at(num) == 0) {
                element_counts.erase(num);
            } 

            // get double value for current element
            int double_num = num * 2;

            // check if map contains double_num
            if (element_counts.contains(double_num)) {
                // remove, if last occurrence
                if (element_counts.at(double_num) == 1) {
                    element_counts.erase(double_num);
                } else {
                    element_counts.at(double_num)--;
                }
            } else {
                // unable to reorder arr
                return false;
            }
        }

        // we are able to reorder arr
        return true;
    }
    bool canReorderDoubled(vector<int>& arr) {
        // store frequencies of each number in a map
        std::unordered_map<int, int> element_counts;

        // loop through arr and add elements to map
        for (unsigned int i = 0; i < arr.size(); i++) {
            int num = arr.at(i);
            // check if map already contains num
            if (element_counts.contains(num)) {
                element_counts.at(num)++;
            } else {
                element_counts.insert({num, 1});
            }
        }

        // sort arr in increasing order
        std::sort(arr.begin(), arr.end());

        // add values to positive and negative lists
        std::vector<int> positives;
        std::vector<int> negatives;
        for (unsigned int i = 0; i < arr.size(); i++) {
            int num = arr.at(i);
            if (num < 0) {
                negatives.insert(negatives.begin(), num);
            } else {
                positives.push_back(num);
            }
        }

        // check both positive and negative, return result
        return containsDuplicates(positives, element_counts) &&
               containsDuplicates(negatives, element_counts);
    }
};
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