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Grokking the Coding Interview Patterns in C#

Solution: Remove Duplicate Letters

Understand how to apply a greedy, stack-based approach to remove duplicate letters from a string, ensuring each letter appears once and the result is lexicographically smallest. This lesson guides you through managing character frequencies, using a stack and set for efficient solution building, and explains the time and space complexity of the algorithm.

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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 ...