LeetCode 316. Remove Duplicate Letters (javascript)

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.

Note: This question is the same as 1081: https://leetcode.com/problems/smallest-subsequence-of-distinct-characters/

Example 1:

Input: s = "bcabc"
Output: "abc"

Example 2:

Input: s = "cbacdcbc"
Output: "acdb"

Constraints:

1 <= s.length <= 10⁴
s consists of lowercase English letters.

Idea:

Hash + Stack

Solution:

/**
 * @param {string} s
 * @return {string}
 */
var removeDuplicateLetters = function(s) {
    let chars = s.split('');
    // record the index of last occurrence for each character
    let lastIndex = new Map();
    for (let i = 0; i < chars.length; i++) {
        lastIndex.set(chars[i], i);
    }
    let stack = [];
    let used = new Map();
    for (let i = 0; i < chars.length; i++) {
        // if the current character has been used, skip it
        if (used.has(chars[i]) && used.get(chars[i])) continue;
        
        // if the current character is smaller than stack[stack.length - 1]
        // and the last index of stack[stack.length - 1] is larger than i, it means we can use it later,
        // so we pop it out and mark used as false
        while (stack.length !== 0 && chars[i] < stack[stack.length - 1] && lastIndex.get(stack[stack.length - 1]) > i)
            used.set(stack.pop(), false);
        
        stack.push(chars[i]);
        used.set(chars[i], true);
    }
    let ans = [];
    while (stack.length !== 0) ans.push(stack.pop());
    return ans.reverse().join('');
};