<p>Given a binary array <code>nums</code>, return <em>the maximum length of a contiguous subarray with an equal number of </em><code>0</code><em> and </em><code>1</code>.</p>

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

<pre>
<strong>Input:</strong> nums = [0,1]
<strong>Output:</strong> 2
<strong>Explanation:</strong> [0, 1] is the longest contiguous subarray with an equal number of 0 and 1.
</pre>

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

<pre>
<strong>Input:</strong> nums = [0,1,0]
<strong>Output:</strong> 2
<strong>Explanation:</strong> [0, 1] (or [1, 0]) is a longest contiguous subarray with equal number of 0 and 1.
</pre>

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

<ul>
	<li><code>1 &lt;= nums.length &lt;= 10<sup>5</sup></code></li>
	<li><code>nums[i]</code> is either <code>0</code> or <code>1</code>.</li>
</ul>

class Solution {
public:
    int findMaxLength(vector<int>& nums) {
        const int n = nums.size();
        std::unordered_map<int, int> diff_to_index_map{{0, -1}};
        auto difference = 0;
        auto ret = 0;
        for (auto i = 0; i < n; ++i) {
            const auto num = nums[i];
            if (num == 1) {
                ++difference;
            } else {
                --difference;
            }
            const auto it = diff_to_index_map.find(difference);
            if (it != diff_to_index_map.end()) {
                ret = std::max(ret, i - it->second);
            } else {
                diff_to_index_map.try_emplace(difference, i);
            }
        }
        return ret;        
    }
};
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