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
class Solution {
public:
    string removeDuplicateLetters(string s) {
        int n = s.size();
        int last[26] = {0};
        for (int i = 0; i < n; ++i) {
            last[s[i] - 'a'] = i;
        }
        string ans;
        int mask = 0;
        for (int i = 0; i < n; ++i) {
            char c = s[i];
            if ((mask >> (c - 'a')) & 1) {
                continue;
            }
            while (!ans.empty() && ans.back() > c && last[ans.back() - 'a'] > i) {
                mask ^= 1 << (ans.back() - 'a');
                ans.pop_back();
            }
            ans.push_back(c);
            mask |= 1 << (c - 'a');
        }
        return ans;
    }
};