76 字母卡牌2

贪心，同样是用数组存储位置

#include <iostream>
#include <string>

using namespace::std;
using std::cout;
using std::cin; 

int main() 
{
    string s;
    cin >> s;
    int cnt[26] = {0}; 
    for (char c : s) 
	{
        cnt[c-'a']++;
    }
    bool vis[26] = {false}; 
    string ans = "";
    for (char c : s) 
	{
        cnt[c-'a']--; 
        if (vis[c-'a']) 
		{
        	continue; 
		}
        while (!ans.empty() && ans.back() > c && cnt[ans.back()-'a'] > 0) 
		{
            vis[ans.back()-'a'] = false; 
            ans.pop_back();
        }
        ans += c; 
        vis[c-'a'] = true; 
    }
    std::cout << ans;
    return 0;
}