Score : $300$ points

问题描述

已知一个由小写英文字母组成的字符串 $S$。判断在 $S$ 开头添加任意数量（可能为零）的 a 是否能使它成为一个回文串。

这里，长度为 $N$ 的字符串 $A=A_1A_2\ldots A_N$ 被称为回文串，当且仅当对于所有满足 $1\leq i\leq N$ 的 $i$，有 $A_i=A_{N+1-i}$。

以上为通义千问 qwen-max 翻译，仅供参考。

Problem Statement

Given is a string $S$ consisting of lowercase English letters. Determine whether adding some number of a's (possibly zero) at the beginning of $S$ can make it a palindrome.

Here, a string of length $N$, $A=A_1A_2\ldots A_N$, is said to be a palindrome when $A_i=A_{N+1-i}$ for every $1\leq i\leq N$.

Constraints

• $1 \leq \lvert S \rvert \leq 10^6$
• $S$ consists of lowercase English letters.

Input

Input is given from Standard Input in the following format:

$S$

Output

If adding some number of a's (possibly zero) at the beginning of $S$ can make it a palindrome, print Yes; otherwise, print No.

Sample Input 1

kasaka

Sample Output 1

Yes

By adding one a at the beginning of kasaka, we have akasaka, which is a palindrome, so Yes should be printed.

Sample Input 2

atcoder

Sample Output 2

No

Adding any number of a's at the beginning of atcoder does not make it a palindrome.

Sample Input 3

php

Sample Output 3

Yes

php itself is a palindrome. Adding zero a's at the beginning of $S$ is allowed, so Yes should be printed.

update @ 2024/3/10 10:15:20