Count Complete Substrings
<p>You are given a string <code>word</code> and an integer <code>k</code>.</p>

<p>A substring <code>s</code> of <code>word</code> is <strong>complete</strong> if:</p>

<ul>
	<li>Each character in <code>s</code> occurs <strong>exactly</strong> <code>k</code> times.</li>
	<li>The difference between two adjacent characters is <strong>at most</strong> <code>2</code>. That is, for any two adjacent characters <code>c1</code> and <code>c2</code> in <code>s</code>, the absolute difference in their positions in the alphabet is <strong>at most</strong> <code>2</code>.</li>
</ul>

<p>Return <em>the number of <strong>complete </strong>substrings of</em> <code>word</code>.</p>

<p>A <strong>substring</strong> is a <strong>non-empty</strong> contiguous sequence of characters in a string.</p>

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

<pre>
<strong>Input:</strong> word = &quot;igigee&quot;, k = 2
<strong>Output:</strong> 3
<strong>Explanation:</strong> The complete substrings where each character appears exactly twice and the difference between adjacent characters is at most 2 are: <u><strong>igig</strong></u>ee, igig<u><strong>ee</strong></u>, <u><strong>igigee</strong></u>.
</pre>

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

<pre>
<strong>Input:</strong> word = &quot;aaabbbccc&quot;, k = 3
<strong>Output:</strong> 6
<strong>Explanation:</strong> The complete substrings where each character appears exactly three times and the difference between adjacent characters is at most 2 are: <strong><u>aaa</u></strong>bbbccc, aaa<u><strong>bbb</strong></u>ccc, aaabbb<u><strong>ccc</strong></u>, <strong><u>aaabbb</u></strong>ccc, aaa<u><strong>bbbccc</strong></u>, <u><strong>aaabbbccc</strong></u>.
</pre>

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

<ul>
	<li><code>1 &lt;= word.length &lt;= 10<sup>5</sup></code></li>
	<li><code>word</code> consists only of lowercase English letters.</li>
	<li><code>1 &lt;= k &lt;= word.length</code></li>
</ul>
Hash Table; String; Sliding Window
func countCompleteSubstrings(word string, k int) (ans int) {
	n := len(word)
	f := func(s string) (ans int) {
		m := len(s)
		for i := 1; i <= 26 && i*k <= m; i++ {
			l := i * k
			cnt := [26]int{}
			for j := 0; j < l; j++ {
				cnt[int(s[j]-'a')]++
			}
			freq := map[int]int{}
			for _, x := range cnt {
				if x > 0 {
					freq[x]++
				}
			}
			if freq[k] == i {
				ans++
			}
			for j := l; j < m; j++ {
				a := int(s[j] - 'a')
				b := int(s[j-l] - 'a')
				freq[cnt[a]]--
				cnt[a]++
				freq[cnt[a]]++

				freq[cnt[b]]--
				cnt[b]--
				freq[cnt[b]]++

				if freq[k] == i {
					ans++
				}
			}
		}
		return
	}
	for i := 0; i < n; {
		j := i + 1
		for j < n && abs(int(word[j])-int(word[j-1])) <= 2 {
			j++
		}
		ans += f(word[i:j])
		i = j
	}
	return
}

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