316\. Remove Duplicate Letters

Medium

Given a string `s`, remove duplicate letters so that every letter appears once and only once. You must make sure your result is **the smallest in lexicographical order** among all possible results.

**Example 1:**

**Input:** s = "bcabc"

**Output:** "abc" 

**Example 2:**

**Input:** s = "cbacdcbc"

**Output:** "acdb" 

**Constraints:**

*   <code>1 <= s.length <= 10<sup>4</sup></code>
*   `s` consists of lowercase English letters.

**Note:** This question is the same as 1081.