C++ 代码

#include <iostream>
#include <stack>
using std::cout, std::cin, std::endl;
using std::stack, std::string;

int main()
{
    string str;
    while (cin >> str)
    {
        stack<char> Stack;
        int i = 0;
        char max = 'a';     //用于标记该字母前的所有字母已经入栈，['a', max)已入栈
        while (str[i])
        {       //遍历字符串
            while (str[i] >= max)       //如果遍历的字符还未入栈，则将其与其前面所有字符入栈
                Stack.push(max++);
            if (str[i++] != Stack.top()) break;     //匹配遍历字符与栈顶字符是否一致
            Stack.pop();
        }
        if (Stack.empty())
            cout << "yes" << endl;
        else
            cout << "no" << endl;
    }

    return 0;
}