Leetcode #2434: Using a Robot to Print the Lexicographically Smallest String

In this guide, we solve Leetcode #2434 Using a Robot to Print the Lexicographically Smallest String in Python and focus on the core idea that makes the solution efficient.

You will see the intuition, the step-by-step method, and a clean Python implementation you can use in interviews.

Leetcode

Problem Statement

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.

Quick Facts

Difficulty: Medium
Premium: No
Tags: Stack, Greedy, Hash Table, String

Intuition

Fast membership checks and value lookups are the heart of this problem, which makes a hash map the natural choice.

By storing what we have already seen (or counts/indexes), we can answer the question in one pass without backtracking.

Approach

Scan the input once, using the map to detect when the condition is satisfied and to update state as you go.

This keeps the solution linear while remaining easy to explain in an interview setting.

Steps:

Initialize a hash map for seen items or counts.
Iterate through the input, querying/updating the map.
Return the first valid result or the final computed value.

Example

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="".

Python Solution

class Solution:
    def robotWithString(self, s: str) -> str:
        cnt = Counter(s)
        ans = []
        stk = []
        mi = 'a'
        for c in s:
            cnt[c] -= 1
            while mi < 'z' and cnt[mi] == 0:
                mi = chr(ord(mi) + 1)
            stk.append(c)
            while stk and stk[-1] <= mi:
                ans.append(stk.pop())
        return ''.join(ans)

Complexity

The time complexity is $O(n + |\Sigma|)$ , and the space complexity is $O(n)$ , where $n$ is the length of the string $s$ and $|\Sigma|$ is the size of the character set, which is $26$ in this problem. The space complexity is $O(n)$ , where $n$ is the length of the string $s$ and $|\Sigma|$ is the size of the character set, which is $26$ in this problem.

Edge Cases and Pitfalls

Watch for boundary values, empty inputs, and duplicate values where applicable. If the problem involves ordering or constraints, confirm the invariant is preserved at every step.

Summary

This Python solution focuses on the essential structure of the problem and keeps the implementation interview-friendly while meeting the constraints.

Approach

Scan the input once, using the map to detect when the condition is satisfied and to update state as you go.

This keeps the solution linear while remaining easy to explain in an interview setting.

Steps:

Initialize a hash map for seen items or counts.

Iterate through the input, querying/updating the map.

Return the first valid result or the final computed value.

Python Solution

class Solution:
    def robotWithString(self, s: str) -> str:
        cnt = Counter(s)
        ans = []
        stk = []
        mi = 'a'
        for c in s:
            cnt[c] -= 1
            while mi < 'z' and cnt[mi] == 0:
                mi = chr(ord(mi) + 1)
            stk.append(c)
            while stk and stk[-1] <= mi:
                ans.append(stk.pop())
        return ''.join(ans)

Complexity

The time complexity is

O(n + |\Sigma|)

, and the space complexity is

O(n)

, where

n

is the length of the string

s

and

|\Sigma|

is the size of the character set, which is

26

in this problem. The space complexity is

O(n)

, where

n

is the length of the string

s

and

|\Sigma|

is the size of the character set, which is

26

in this problem.

Leetcode #2434: Using a Robot to Print the Lexicographically Smallest String

Problem Statement

Quick Facts

Intuition

Approach

Example

Python Solution

Complexity

Edge Cases and Pitfalls

Summary

Ace your next coding interview

Leetcode #2434: Using a Robot to Print the Lexicographically Smallest String

Problem Statement

Quick Facts

Intuition

Approach

Example

Python Solution

Complexity

Edge Cases and Pitfalls

Summary

Ace your next coding interview