<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(std::vector<int> &nums)
    {

        int sum = 0;
        int max = 0;
        appearances_[0].first = -1;
        for (int i = 0; i < nums.size(); ++i)
        {
            sum += nums[i] * 2 + -1;
            if (appearances_.find(sum) == appearances_.end())
            {
                appearances_[sum].first = i;
            }
            else
            {
                int distance = i - appearances_[sum].first;
                if (distance > max)
                {
                    max = distance;
                }

                appearances_[sum].last = i;
            }

        }

        return max;
    }

private:
    struct Appearance
    {
        int first;
        int last;
    };

    std::unordered_map<int, Appearance> appearances_;
};
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