316 Remove Duplicate Letters 去除重复字母

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.

Note:
This question is the same as 1081

Example:

Example 1:

Input: s = "bcabc"
Output: "abc"

Example 2:

Input: s = "cbacdcbc"
Output: "acdb"

Constraints:

1 <= s.length <= 10^4
s consists of lowercase English letters.

题目描述:
给你一个字符串 s ，请你去除字符串中重复的字母，使得每个字母只出现一次。需保证 返回结果的字典序最小（要求不能打乱其他字符的相对位置）。

注意：
该题与 1081 相同

示例 :

示例 1：

输入：s = "bcabc"
输出："abc"

示例 2：

输入：s = "cbacdcbc"
输出："acdb"

提示：

1 <= s.length <= 10^4
s 由小写英文字母组成

思路:

单调栈
先统计字符串中的字符的个数
遍历字符串, 减去对应的字符
访问过的字符就跳过
如果栈中的元素比遍历的字符要大, 尝试弹出
如果之后没有该元素, 就不弹出, 否则全部弹出直到遍历元素为最小元素
时间复杂度O(n), 空间复杂度O(n)

代码:
C++:

class Solution 
{
public:
    string removeDuplicateLetters(string s) 
    {
        vector<int> count(128);
        vector<bool> visited(128, false);
        for (auto &c : s) ++count[c];
        string result = "0";
        for (auto &c : s)
        {
            --count[c];
            if (visited[c]) continue;
            while (c < result.back() and count[result.back()])
            {
                visited[result.back()] = false;
                result.pop_back();
            }
            result += c;
            visited[c] = true;
        }
        return result.substr(1);
    }
};

Java:

class Solution {
    public String removeDuplicateLetters(String s) {
        Stack<Character> stack = new Stack<>();
        boolean visited[] = new boolean[128];
        int count[] = new int[128];
        for (char c : s.toCharArray()) ++count[c];
        for (char c : s.toCharArray()) {
            --count[c];
            if (visited[c]) continue;
            while (!stack.isEmpty() && stack.peek() > c) {
                if (count[stack.peek()] == 0) break;
                visited[stack.pop()] = false;
            }
            stack.push(c);
            visited[c] = true;
        }
        StringBuilder sb = new StringBuilder();
        while (!stack.isEmpty()) sb.append(stack.pop());
        return sb.reverse().toString();
    }
}

Python:

class Solution:
    def removeDuplicateLetters(self, s: str) -> str:
        for a in sorted(set(s)):
            temp = s[s.index(a):]
            if len(set(temp)) == len(set(s)):
                return a + self.removeDuplicateLetters(temp.replace(a, ""))
        return ""