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2434. Using a Robot to Print the Lexicographically Smallest String

Difficulty Topics

Description

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.

Solution

using-a-robot-to-print-the-lexicographically-smallest-string.py
class Solution:
    def robotWithString(self, s: str) -> str:
        N = len(s)
        res = []
        stack = []
        count = Counter(s)

        for x in s:
            stack.append(x)

            if count[x] == 1:
                del count[x]
            else:
                count[x] -= 1

            while stack and count and stack[-1] <= min(count):
                res.append(stack.pop())

        if stack:
            res += stack[::-1]

        return "".join(res)