Leetcode | Remove Duplicate Letters_remove duplicate letters leetcode-优快云博客

本文链接：https://blog.youkuaiyun.com/AEhaojiu/article/details/53544062

本文介绍了一种使用栈来去除字符串中重复字母的方法，并确保剩余字母序列的字典序最小。通过跟踪字符出现次数及已访问状态，算法可以高效地处理输入，返回符合要求的结果。

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question

Given a string which contains only lowercase letters, remove duplicate letters so that every letter appear once and only once. You must make sure your result is the smallest in lexicographical order among all possible results.

Example:
Given “bcabc”
Return “abc”

Given “cbacdcbc”
Return “acdb”

solution

//用数组标记字符出现的次数用数组标记字符是否被访问(代表是否已经找到最优值)
//用栈保存结果当有新的字符入栈时判断在栈中的比入栈字符大的是否在栈后出现若是则出栈以此循环可得到结果

code

public static String removeDuplicateLetters(String s) {

    Stack<Character> stack=new Stack<Character>();
    int []res=new int[26];
    boolean [] visited=new boolean[26];

    int len=s.length();
    for(int i=0; i<len; ++i)
    {
        res[s.charAt(i)-'a']++;

    }

    for(char ch:s.toCharArray())
    {

        //System.out.println("ch: "+ch);
        res[ch-'a']--;
        if(visited[ch-'a'])
            continue;

        while(!stack.isEmpty() && stack.peek() > ch && res[stack.peek()-'a']!=0 )
        {

            visited[stack.pop()-'a']=false;

        }

        stack.push(ch);
        //System.out.println("push: "+ch);
        visited[ch-'a']=true;



    }

    StringBuilder sb = new StringBuilder();

    while(!stack.isEmpty())
    {

        sb.insert(0,stack.pop());

    }



    return sb.toString();

}