Get Equal Substrings Within Budget
<p>You are given two strings <code>s</code> and <code>t</code> of the same length and an integer <code>maxCost</code>.</p>

<p>You want to change <code>s</code> to <code>t</code>. Changing the <code>i<sup>th</sup></code> character of <code>s</code> to <code>i<sup>th</sup></code> character of <code>t</code> costs <code>|s[i] - t[i]|</code> (i.e., the absolute difference between the ASCII values of the characters).</p>

<p>Return <em>the maximum length of a substring of </em><code>s</code><em> that can be changed to be the same as the corresponding substring of </em><code>t</code><em> with a cost less than or equal to </em><code>maxCost</code>. If there is no substring from <code>s</code> that can be changed to its corresponding substring from <code>t</code>, return <code>0</code>.</p>

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

<pre>
<strong>Input:</strong> s = &quot;abcd&quot;, t = &quot;bcdf&quot;, maxCost = 3
<strong>Output:</strong> 3
<strong>Explanation:</strong> &quot;abc&quot; of s can change to &quot;bcd&quot;.
That costs 3, so the maximum length is 3.
</pre>

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

<pre>
<strong>Input:</strong> s = &quot;abcd&quot;, t = &quot;cdef&quot;, maxCost = 3
<strong>Output:</strong> 1
<strong>Explanation:</strong> Each character in s costs 2 to change to character in t,  so the maximum length is 1.
</pre>

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

<pre>
<strong>Input:</strong> s = &quot;abcd&quot;, t = &quot;acde&quot;, maxCost = 0
<strong>Output:</strong> 1
<strong>Explanation:</strong> You cannot make any change, so the maximum length is 1.
</pre>

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

<ul>
	<li><code>1 &lt;= s.length &lt;= 10<sup>5</sup></code></li>
	<li><code>t.length == s.length</code></li>
	<li><code>0 &lt;= maxCost &lt;= 10<sup>6</sup></code></li>
	<li><code>s</code> and <code>t</code> consist of only lowercase English letters.</li>
</ul>
String; Binary Search; Prefix Sum; Sliding Window
func equalSubstring(s string, t string, maxCost int) int {
	n := len(s)
	f := make([]int, n+1)
	for i, a := range s {
		f[i+1] = f[i] + abs(int(a)-int(t[i]))
	}
	check := func(x int) bool {
		for i := 0; i+x-1 < n; i++ {
			if f[i+x]-f[i] <= maxCost {
				return true
			}
		}
		return false
	}
	l, r := 0, n
	for l < r {
		mid := (l + r + 1) >> 1
		if check(mid) {
			l = mid
		} else {
			r = mid - 1
		}
	}
	return l
}

func abs(x int) int {
	if x < 0 {
		return -x
	}
	return x
}