316. Remove Duplicate Letters

Problem

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

Solution

关键是要发现一个规律 ： 从前往后扫描时，当前字符取舍在于 当前结果字符串出现比它大的字符 在后面的字符串是否还存在多余的，如果存在，那就把前面的删去，用后面的；否则就keep 当前的

用 count 和 added 两个数组来辅助才行。

refer   https://leetcode.com/discuss/83225/clean-easy-understand-java-stack-solution-with-explanation

class Solution {
public:
    string removeDuplicateLetters(string s) {
        const int N = s.size();
        
        if( N < 2 ) return s;
        vector<int> count( 26, 0 );
        for(char c : s) {
            count[c-'a']++;
        }
        vector<bool> added(26, false);
        string rst;
        for(char c : s) {
            count[c-'a']--;
            if(added[c-'a']) continue;
            
            while(!rst.empty() && rst.back() > c && count[rst.back() -'a'] > 0) {
                added[rst.back() - 'a'] = false;
                rst.pop_back();
            }
            rst.push_back(c);
            added[c-'a'] = true;
        }
        return rst;
        
    }
};