2434. Using a Robot to Print the Lexicographically Smallest String

Problem Statement

2434. Using a Robot to Print the Lexicographically Smallest String

Medium

You are given a string s and a robot that currently holds an empty string t. Apply one of the following operations until s and t are both empty:

Remove the first character of a string s and give it to the robot. The robot will append this character to the string t.
Remove the last character of a string t and give it to the robot. The robot will write this character on paper.

Return the lexicographically smallest string that can be written on the paper.

Example 1:

Input: s = "zza"
Output: "azz"
Explanation: Let p denote the written string.
Initially p="", s="zza", t="".
Perform first operation three times p="", s="", t="zza".
Perform second operation three times p="azz", s="", t="".

Example 2:

Input: s = "bac"
Output: "abc"
Explanation: Let p denote the written string.
Perform first operation twice p="", s="c", t="ba". 
Perform second operation twice p="ab", s="c", t="". 
Perform first operation p="ab", s="", t="c". 
Perform second operation p="abc", s="", t="".

Example 3:

Input: s = "bdda"
Output: "addb"
Explanation: Let p denote the written string.
Initially p="", s="bdda", t="".
Perform first operation four times p="", s="", t="bdda".
Perform second operation four times p="addb", s="", t="".

Constraints:

1 <= s.length <= 10⁵
s consists of only English lowercase letters.

Java

Source file

class Solution {
    public String robotWithString(String s) {
        Stack<Character> t = new Stack<>();
        StringBuilder sb = new StringBuilder();
        int[] freq = new int[26];
        int n = s.length();
        for (int i = 0; i < n; i++)
            freq[s.charAt(i) - 'a']++;
        for (int i = 0, ptr = 0; i < n; i++) {
            char c = s.charAt(i);
            t.push(c);
            freq[c - 'a']--;
            while (ptr < 26 && freq[ptr] == 0)
                ptr++;
            while (!t.isEmpty() && t.peek() - 'a' <= ptr)
                sb.append(t.pop());
        }
        return sb.toString();
    }
}