Greedy on string
Last updated: February 4, 2026
Quick Overview
Given a string consisting of lowercase letters, implement a greedy algorithm to determine the longest substring that can be formed by removing at most one character, such that the remaining characters are in non-decreasing order. The function should return the length of this longest valid substring.
DoorDash
February 4, 2026112
10
1,734 solved
Given a string consisting of lowercase letters, implement a greedy algorithm to determine the longest substring that can be formed by removing at most one character, such that the remaining characters are in non-decreasing order. The function should return the length of this longest valid substring.
DoorDash uses this problem in the Technical Screen to evaluate your algorithmic thinking. They expect you to discuss multiple approaches, analyze trade-offs between them, and implement the optimal solution with clean, readable code.
What the Interviewer Expects
- Quickly identify the optimal approach and its theoretical basis
- Handle complex algorithm design with multiple interacting components
- Write concise, elegant code under time pressure
- Prove correctness of your approach and discuss alternative solutions
- Optimize beyond the obvious: discuss constant factor improvements
- Address follow-up variations and explain how the solution generalizes
Key Topics to Cover
How to Approach This
- Clarify input constraints and edge cases before writing code.
- Walk through your approach verbally and confirm with the interviewer before coding.
- Start with a brute force solution, then optimize. Mention time and space complexity.
- Test your solution with examples, including edge cases like empty input or duplicates.
- Consider common patterns: sliding window, two pointers, hash map, BFS/DFS, dynamic programming.
Possible Follow-up Questions
- Can you optimize the space complexity of your solution?
- How would you parallelize this solution?
- How would you modify your solution to handle streaming input?
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Practice DSA ProblemsSample Answer
Problem Analysis
The problem requires us to find the longest substring that can be made non-decreasing by removing at most one character. A greedy approach is suitable here because we want to maximize the length of th...
Approach
- Initialize two pointers,
leftandright, both starting at the beginning of the string. Therightpointer will be used to expand the substring, while theleftpointer will help in identifyin...
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