Smallest Subsequence of Distinct Characters

Problem statement

Given a string s, return the lexicographically smallest subsequence of s that contains all the distinct characters of s exactly once.

Example 1:

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

Example 2:

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

Constraints:

• 1 <= s.length <= 1000
• s consists of lowercase English letters.

Note: This question is the same as 316: https://leetcode.com/problems/remove-duplicate-letters/

My solution

/**
 * @param {string} s
 * @return {string}
 */
var smallestSubsequence = function(s) {
    const frequency = new Map();
    const visited = new Map();
    
    for (const char of s) {
        if (!frequency.has(char)) {
            frequency.set(char, 1);
            visited.set(char, 0)
        } else {
            frequency.set(char, frequency.get(char) + 1)
        }
    }
    
    const stack = []
    
    for (const char of s) {
        // console.log("stack", stack)
        if (!stack.length) {
            stack.push(char)
            visited.set(char, 1)
        } else {
            let top = stack[stack.length -1]
            
            if (frequency.get(top) > 0 && char < top && !visited.get(char)) {
                while (stack.length && frequency.get(top) > 0 && char < top && !visited.get(char)) {
                    let popped = stack.pop();
                    top = stack [stack.length - 1]
                    visited.set(popped, 0)
                }
                
                stack.push(char);
                visited.set(char, 1)
            } else if (!visited.get(char)){
                stack.push(char);
                visited.set(char, 1)
            }
        }
        
        frequency.set(char, frequency.get(char) - 1)
    }
    
    return stack.join("")
};