316. Remove Duplicate Letters
Description
Given a string s
, remove duplicate letters so that every letter appears once and only once. You must make sure your result is the smallest in lexicographical order among all possible results.
Example 1:
Input: s = "bcabc" Output: "abc"
Example 2:
Input: s = "cbacdcbc" Output: "acdb"
Constraints:
1 <= s.length <= 104
s
consists of lowercase English letters.
Note: This question is the same as 1081: https://leetcode.com/problems/smallest-subsequence-of-distinct-characters/
Solutions
Solution 1: Stack
We use an array last
to record the last occurrence of each character, a stack to save the result string, and an array vis
or an integer variable mask
to record whether the current character is in the stack.
Traverse the string $s$, for each character $c$, if $c$ is not in the stack, we need to check whether the top element of the stack is greater than $c$. If it is greater than $c$ and the top element of the stack will appear later, we pop the top element of the stack and push $c$ into the stack.
Finally, concatenate the elements in the stack into a string and return it as the result.
The time complexity is $O(n)$, and the space complexity is $O(n)$. Where $n$ is the length of the string $s$.
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Solution 2
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