Number of Ways to Split a String
<p>Given a binary string <code>s</code>, you can split <code>s</code> into 3 <strong>non-empty</strong> strings <code>s1</code>, <code>s2</code>, and <code>s3</code> where <code>s1 + s2 + s3 = s</code>.</p>

<p>Return the number of ways <code>s</code> can be split such that the number of ones is the same in <code>s1</code>, <code>s2</code>, and <code>s3</code>. Since the answer may be too large, return it <strong>modulo</strong> <code>10<sup>9</sup> + 7</code>.</p>

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

<pre>
<strong>Input:</strong> s = &quot;10101&quot;
<strong>Output:</strong> 4
<strong>Explanation:</strong> There are four ways to split s in 3 parts where each part contain the same number of letters &#39;1&#39;.
&quot;1|010|1&quot;
&quot;1|01|01&quot;
&quot;10|10|1&quot;
&quot;10|1|01&quot;
</pre>

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

<pre>
<strong>Input:</strong> s = &quot;1001&quot;
<strong>Output:</strong> 0
</pre>

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

<pre>
<strong>Input:</strong> s = &quot;0000&quot;
<strong>Output:</strong> 3
<strong>Explanation:</strong> There are three ways to split s in 3 parts.
&quot;0|0|00&quot;
&quot;0|00|0&quot;
&quot;00|0|0&quot;
</pre>

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

<ul>
	<li><code>3 &lt;= s.length &lt;= 10<sup>5</sup></code></li>
	<li><code>s[i]</code> is either <code>&#39;0&#39;</code> or <code>&#39;1&#39;</code>.</li>
</ul>
Math; String
func numWays(s string) int {
	cnt := 0
	for _, c := range s {
		if c == '1' {
			cnt++
		}
	}
	m := cnt % 3
	if m != 0 {
		return 0
	}
	const mod = 1e9 + 7
	n := len(s)
	if cnt == 0 {
		return (n - 1) * (n - 2) / 2 % mod
	}
	cnt /= 3
	find := func(x int) int {
		t := 0
		for i := 0; ; i++ {
			if s[i] == '1' {
				t++
				if t == x {
					return i
				}
			}
		}
	}
	i1, i2 := find(cnt), find(cnt+1)
	j1, j2 := find(cnt*2), find(cnt*2+1)
	return (i2 - i1) * (j2 - j1) % mod
}