#2434

Medium

auto_awesomeStack-based state management

LeetCode Problem Workspace

Using a Robot to Print the Lexicographically Smallest String

Solve the problem of using a robot to print the lexicographically smallest string with stack-based state management.

Problem Statement

You are given a string s and a robot that holds an initially empty string t. You need to apply the following two operations repeatedly until both strings are empty:

Move the leftmost character of s to the end of t. 2. Move the rightmost character of t to the end of the result string p. Return the lexicographically smallest string p that can be created from these operations.

Examples

Example 1

Input: s = "zza"

Output: "azz"

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"

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"

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 Approach

Stack-Based Operations

Use a stack to manage the string efficiently. The robot should decide whether to move the leftmost character from s to t or the rightmost character from t to the result string based on lexicographical order.

Greedy Strategy

In each operation, choose the lexicographically smallest available character. By comparing the top of the stack (t) with the remaining characters of s, you can ensure that the smallest possible string is formed.

Time and Space Complexity Management

The time complexity is O(n + |Σ|), where n is the length of s, and Σ is the character set. Space complexity is O(n) due to the stack and result string storage.

Complexity Analysis

Metric	Value
Time	O(n +
Space	O(n)

The time complexity of O(n + |Σ|) arises because each operation processes characters either from s or t. The space complexity is O(n) due to the stack storing up to n characters at any point in time.

What Interviewers Usually Probe

Candidate demonstrates strong understanding of greedy algorithms and stack-based data structures.
They are able to prioritize the lexicographically smallest character efficiently.
The candidate can explain and optimize the trade-offs between time complexity and space usage.

Common Pitfalls or Variants

Common pitfalls

Failing to recognize when to choose a character from the stack over s can lead to incorrect results.
Not managing the stack size properly may result in inefficient space usage or wrong lexicographical order.
Misunderstanding the problem's stack-based state management leads to inefficient or incorrect operation sequences.

Follow-up variants

Consider cases where the input string is already sorted or reversed.
What if additional constraints or operations are introduced, such as multiple robot actions?
How would the solution change if the problem involved different character sets or larger strings?

FAQ

How do I apply stack-based state management to solve the "Using a Robot to Print the Lexicographically Smallest String" problem?

Use a stack to manage characters from the input string and robot's temporary string, ensuring the lexicographically smallest string is formed by selecting characters in the right order.

What is the key to solving "Using a Robot to Print the Lexicographically Smallest String"?

The key is using a stack-based approach to make greedy decisions about whether to move a character from the string or the stack to the result string, prioritizing lexicographical order.

How do I handle the lexicographical comparison between characters in "Using a Robot to Print the Lexicographically Smallest String"?

Compare the characters at the top of the stack with those left in the string, and always move the smallest character to the result string to ensure the smallest possible outcome.

What challenges might I face when solving "Using a Robot to Print the Lexicographically Smallest String"?

The main challenge is managing the stack effectively while ensuring the correct lexicographical order, avoiding unnecessary space usage or incorrect string manipulation.

What are the time and space complexities of "Using a Robot to Print the Lexicographically Smallest String"?

The time complexity is O(n + |Σ|), where n is the string length, and the space complexity is O(n) due to the space used by the stack and result string.

terminal

Solution

Solution 1: Greedy + Stack

The problem can be transformed into: given a string sequence, use an auxiliary stack to convert it into the lexicographically smallest string sequence.

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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)

Continue Practicing

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