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Grokking the Coding Interview Patterns in C++

Solution: Remove Duplicate Letters

Understand how to use a greedy stack-based approach to remove duplicate letters from a lowercase string, producing the lexicographically smallest string with unique characters. This lesson guides you through the algorithm, frequency tracking, and stack operations that build the final result efficiently.

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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 ...