Amazon·Software Engineer·Onsite - Coding / Algorithms·Intermediate

IntermediatePrefer not to say

Jul 2026

Summary

Amazon SWE coding round with a string manipulation problem that looks deceptively simple but has some tricky edge cases once you start working through the examples.

Questions Asked (1)

Given a string of digit characters representing box IDs, you can repeatedly pick any digit, remove it, increment it by 1 (capped at 9), and reinsert it anywhere in the string. Return the lexicographically smallest string achievable after any number of such operations.

Algorithms & Data Structures

Author's notes

Spent the first few minutes just staring at the examples trying to figure out what the operation was actually doing in practice.

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AI HintsAI Generated

Suggested Approach

First, recognize that the operation allows incrementing digits and reinserting them anywhere, so the goal is to minimize the string lexicographically. The optimal strategy is to sort the digits in non-decreasing order, but with the twist that we can increment digits up to 9; however, incrementing a digit never helps because it makes it larger. Thus, the lexicographically smallest string is simply the original digits sorted in ascending order.

Pro tip: Clarify that incrementing a digit is never beneficial for lexicographic minimization, as it only increases the digit's value; the operation's real power is the ability to reorder digits arbitrarily. This shows you understand the operation's implications and avoid overcomplicating the solution.

1. Understand the operation

Explain that you can pick any digit, increment it by 1 (capped at 9), and reinsert it anywhere. Note that incrementing increases the digit's value, which is counterproductive for lexicographic minimization.

2. Identify the goal

The goal is to produce the lexicographically smallest string possible. Lexicographic order compares strings character by character, so smaller digits earlier are better.

3. Determine optimal strategy

Since incrementing only makes digits larger, never increment any digit. The only useful action is reordering, which allows any permutation of the original digits.

4. Sort digits ascending

To get the lexicographically smallest permutation, sort the digits in non-decreasing order. This places the smallest digits first.

5. Return result

The sorted string is the answer. Confirm with examples, e.g., '321' becomes '123'.

Key Points to Mention

Incrementing a digit never helps because it increases its value, making the string lexicographically larger.
The operation allows arbitrary reordering of digits, so any permutation of the original multiset is achievable.
The lexicographically smallest permutation of a multiset of digits is the sorted ascending order.
Time complexity is O(n log n) for sorting, or O(n) with counting sort since digits are 0-9.
Edge cases: empty string, all same digits, digits already sorted.
Clarify that the cap at 9 doesn't affect the optimal strategy since we never increment.

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