Maximum Difference Between Even and Odd Frequency II
<p>You are given a string <code>s</code> and an integer <code>k</code>. Your task is to find the <strong>maximum</strong> difference between the frequency of <strong>two</strong> characters, <code>freq[a] - freq[b]</code>, in a <span data-keyword="substring">substring</span> <code>subs</code> of <code>s</code>, such that:</p>

<ul>
	<li><code>subs</code> has a size of <strong>at least</strong> <code>k</code>.</li>
	<li>Character <code>a</code> has an <em>odd frequency</em> in <code>subs</code>.</li>
	<li>Character <code>b</code> has a <strong>non-zero</strong> <em>even frequency</em> in <code>subs</code>.</li>
</ul>

<p>Return the <strong>maximum</strong> difference.</p>

<p><strong>Note</strong> that <code>subs</code> can contain more than 2 <strong>distinct</strong> characters.</p>

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

<div class="example-block">
<p><strong>Input:</strong> <span class="example-io">s = &quot;12233&quot;, k = 4</span></p>

<p><strong>Output:</strong> <span class="example-io">-1</span></p>

<p><strong>Explanation:</strong></p>

<p>For the substring <code>&quot;12233&quot;</code>, the frequency of <code>&#39;1&#39;</code> is 1 and the frequency of <code>&#39;3&#39;</code> is 2. The difference is <code>1 - 2 = -1</code>.</p>
</div>

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

<div class="example-block">
<p><strong>Input:</strong> <span class="example-io">s = &quot;1122211&quot;, k = 3</span></p>

<p><strong>Output:</strong> <span class="example-io">1</span></p>

<p><strong>Explanation:</strong></p>

<p>For the substring <code>&quot;11222&quot;</code>, the frequency of <code>&#39;2&#39;</code> is 3 and the frequency of <code>&#39;1&#39;</code> is 2. The difference is <code>3 - 2 = 1</code>.</p>
</div>

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

<div class="example-block">
<p><strong>Input:</strong> <span class="example-io">s = &quot;110&quot;, k = 3</span></p>

<p><strong>Output:</strong> <span class="example-io">-1</span></p>
</div>

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

<ul>
	<li><code>3 &lt;= s.length &lt;= 3 * 10<sup>4</sup></code></li>
	<li><code>s</code> consists only of digits <code>&#39;0&#39;</code> to <code>&#39;4&#39;</code>.</li>
	<li>The input is generated that at least one substring has a character with an even frequency and a character with an odd frequency.</li>
	<li><code>1 &lt;= k &lt;= s.length</code></li>
</ul>
String; Enumeration; Prefix Sum; Sliding Window
func maxDifference(s string, k int) int {
	n := len(s)
	inf := math.MaxInt32 / 2
	ans := -inf

	for a := 0; a < 5; a++ {
		for b := 0; b < 5; b++ {
			if a == b {
				continue
			}
			curA, curB := 0, 0
			preA, preB := 0, 0
			t := [2][2]int{{inf, inf}, {inf, inf}}
			l := -1

			for r := 0; r < n; r++ {
				if s[r] == byte('0'+a) {
					curA++
				}
				if s[r] == byte('0'+b) {
					curB++
				}

				for r-l >= k && curB-preB >= 2 {
					t[preA&1][preB&1] = min(t[preA&1][preB&1], preA-preB)
					l++
					if s[l] == byte('0'+a) {
						preA++
					}
					if s[l] == byte('0'+b) {
						preB++
					}
				}

				ans = max(ans, curA-curB-t[curA&1^1][curB&1])
			}
		}
	}

	return ans
}