Solution: Remove Duplicate Letters

Explore how to remove duplicate letters from a string by using a stack-based greedy approach that ensures each character appears once and the result is lexicographically smallest. Understand the key steps including frequency counting, maintaining visited sets, and stack operations to build the final string efficiently with linear time complexity.

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Statement
Solution
- Time complexity
- Space complexity

Statement

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

Each letter appears only once in the resulting string.
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 prefix of b, then a is considered smaller.

Constraints:

$1 \leq$ s.length ...