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Grokking the Coding Interview Patterns in Python

Solution: Remove Duplicate Letters

Explore how to use a stack-based greedy approach to remove duplicate letters from a string while ensuring the final result is the smallest lexicographically. Understand how to track frequencies and manage a stack to build the solution in linear time and constant space complexity.

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Statement

You are given a string, s, consisting only of lowercase English letters. Your task is to remove duplicate letters so that:

  1. Each letter appears only once in the resulting string.

  2. The resulting string is the smallest in lexicographical order among all possible results that satisfy the above condition.

Return the final string.

A string, a, is considered lexicographically smaller than string b if:

  • In the first position where they differ, the character in a appears earlier in the alphabet than in b.

  • If a is a ...