Smallest Subsequence of Distinct Characters
<p>Given a string <code>s</code>, return <em>the </em><span data-keyword="lexicographically-smaller-string"><em>lexicographically smallest</em></span> <span data-keyword="subsequence-string"><em>subsequence</em></span><em> of</em> <code>s</code> <em>that contains all the distinct characters of</em> <code>s</code> <em>exactly once</em>.</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;= 1000</code></li>
	<li><code>s</code> consists of lowercase English letters.</li>
</ul>

<p>&nbsp;</p>
<strong>Note:</strong> This question is the same as 316: <a href="https://leetcode.com/problems/remove-duplicate-letters/" target="_blank">https://leetcode.com/problems/remove-duplicate-letters/</a>
Stack; Greedy; String; Monotonic Stack
function smallestSubsequence(s: string): string {
    const f = (c: string): number => c.charCodeAt(0) - 'a'.charCodeAt(0);
    const last: number[] = new Array(26).fill(0);
    for (const [i, c] of [...s].entries()) {
        last[f(c)] = i;
    }
    const stk: string[] = [];
    let mask = 0;
    for (const [i, c] of [...s].entries()) {
        const x = f(c);
        if ((mask >> x) & 1) {
            continue;
        }
        while (stk.length && stk[stk.length - 1] > c && last[f(stk[stk.length - 1])] > i) {
            mask ^= 1 << f(stk.pop()!);
        }
        stk.push(c);
        mask |= 1 << x;
    }
    return stk.join('');
}
