[LeetCode] 316. Remove Duplicate Letters 移除重復字母

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"

Credits:
Special thanks to @dietpepsi for adding this problem and creating all test cases.

給定一個只含有小寫字母的字符串，移除重復的字符，每種字符只出現1次，返回字符串是按字典順序中最小的組合。

解法：貪心算法Greedy，用1個HashMap或者數組統計出現的字符。然后再遍歷一遍數組, 假設position = 0，每次找到字符比position的小就更新position，同時也更新count，當count[i] == 0的時候，這個字符就是我們要找的結果字符串里的第一個字符。之后因為其他字符的count還都 > 1，繼續在s.substring(position + 1)的子串里遞歸查找第二個字符，注意要在這個子串里把第一個字符清除掉。

Java：

public class Solution {
    public String removeDuplicateLetters(String s) {
        if(s == null || s.length() == 0) {
            return s;
        }
        
        int[] count = new int[26];
        char[] res = new char[26];
        int len = s.length();
        for(int i = 0; i < s.length(); i++) {
            count[s.charAt(i) - 'a']++; 
        }
        
        int pos = 0;
        for(int i = 0; i < len; i++) {
            if(s.charAt(i) < s.charAt(pos)) {
                pos = i;
            }
            count[s.charAt(i) - 'a']--;     
            if(count[s.charAt(i) - 'a'] == 0) {         // found first minimum char
                break;
            }
        }
        
        String charToRemove = String.valueOf(s.charAt(pos));
        return charToRemove + removeDuplicateLetters(s.substring(pos + 1).replaceAll(charToRemove, ""));
    }
}　　

Python:

class Solution(object):
    def removeDuplicateLetters(self, s):
        """
        :type s: str
        :rtype: str
        """
        remaining = collections.defaultdict(int)
        for c in s:
            remaining[c] += 1

        in_stack, stk = set(), []
        for c in s:
            if c not in in_stack:
                while stk and stk[-1] > c and remaining[stk[-1]]:
                    in_stack.remove(stk.pop())
                stk += c
                in_stack.add(c)
            remaining[c] -= 1
        return "".join(stk)　　

C++：

class Solution {
public:
    string removeDuplicateLetters(string s) {
        int m[256] = {0}, visited[256] = {0};
        string res = "0";
        for (auto a : s) ++m[a];
        for (auto a : s) {
            --m[a];
            if (visited[a]) continue;
            while (a < res.back() && m[res.back()]) {
                visited[res.back()] = 0;
                res.pop_back();
            }
            res += a;
            visited[a] = 1;
        }
        return res.substr(1);
    }
};　

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