Remove Duplicate Letters
<p>Given a string <code>s</code>, remove duplicate letters so that every letter appears once and only once. You must make sure your result is <span data-keyword="lexicographically-smaller-string"><strong>the smallest in lexicographical order</strong></span> among all possible results.</p>

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

<pre>
<strong>Input:</strong> s = &quot;bcabc&quot;
<strong>Output:</strong> &quot;abc&quot;
</pre>

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

<pre>
<strong>Input:</strong> s = &quot;cbacdcbc&quot;
<strong>Output:</strong> &quot;acdb&quot;
</pre>

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

<ul>
	<li><code>1 &lt;= s.length &lt;= 10<sup>4</sup></code></li>
	<li><code>s</code> consists of lowercase English letters.</li>
</ul>

<p>&nbsp;</p>
<p><strong>Note:</strong> This question is the same as 1081: <a href="https://leetcode.com/problems/smallest-subsequence-of-distinct-characters/" target="_blank">https://leetcode.com/problems/smallest-subsequence-of-distinct-characters/</a></p>
Stack; Greedy; String; Monotonic Stack
func removeDuplicateLetters(s string) string {
	last := make([]int, 26)
	for i, c := range s {
		last[c-'a'] = i
	}
	stk := []rune{}
	vis := make([]bool, 128)
	for i, c := range s {
		if vis[c] {
			continue
		}
		for len(stk) > 0 && stk[len(stk)-1] > c && last[stk[len(stk)-1]-'a'] > i {
			vis[stk[len(stk)-1]] = false
			stk = stk[:len(stk)-1]
		}
		stk = append(stk, c)
		vis[c] = true
	}
	return string(stk)
}