Find the Longest Balanced Substring of a Binary String
<p>You are given a binary string <code>s</code> consisting only of zeroes and ones.</p>

<p>A substring of <code>s</code> is considered balanced if<strong> all zeroes are before ones</strong> and the number of zeroes is equal to the number of ones inside the substring. Notice that the empty substring is considered a balanced substring.</p>

<p>Return <em>the length of the longest balanced substring of </em><code>s</code>.</p>

<p>A <b>substring</b> is a contiguous sequence of characters within a string.</p>

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

<pre>
<strong>Input:</strong> s = &quot;01000111&quot;
<strong>Output:</strong> 6
<strong>Explanation:</strong> The longest balanced substring is &quot;000111&quot;, which has length 6.
</pre>

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

<pre>
<strong>Input:</strong> s = &quot;00111&quot;
<strong>Output:</strong> 4
<strong>Explanation:</strong> The longest balanced substring is &quot;0011&quot;, which has length 4.&nbsp;
</pre>

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

<pre>
<strong>Input:</strong> s = &quot;111&quot;
<strong>Output:</strong> 0
<strong>Explanation:</strong> There is no balanced substring except the empty substring, so the answer is 0.
</pre>

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

<ul>
	<li><code>1 &lt;= s.length &lt;= 50</code></li>
	<li><code>&#39;0&#39; &lt;= s[i] &lt;= &#39;1&#39;</code></li>
</ul>
String
func findTheLongestBalancedSubstring(s string) (ans int) {
	n := len(s)
	check := func(i, j int) bool {
		cnt := 0
		for k := i; k <= j; k++ {
			if s[k] == '1' {
				cnt++
			} else if cnt > 0 {
				return false
			}
		}
		return cnt*2 == j-i+1
	}
	for i := 0; i < n; i++ {
		for j := i + 1; j < n; j++ {
			if check(i, j) {
				ans = max(ans, j-i+1)
			}
		}
	}
	return
}