F

Mike has a string s consisting of only lowercase English letters. He wants to change exactly one character from the string so that the resulting one is a palindrome.

A palindrome is a string that reads the same backward as forward, for example strings “z”, “aaa”, “aba”, “abccba” are palindromes, but strings “codeforces”, “reality”, “ab” are not.

Input
The first and single line contains string s (1 ≤ |s| ≤ 15).

Output
Print “YES” (without quotes) if Mike can change exactly one character so that the resulting string is palindrome or “NO” (without quotes) otherwise.

Examples
Input
abccaa
Output
YES
Input
abbcca
Output
NO
Input
abcda
Output
YES

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
int main()
{
    int sum=0,n,i;
    char a[16];
    scanf("%s",a);
    n=strlen(a);
    for(i=0;i<n-i-1;i++)
    {
        if(a[i]!=a[n-1-i])
        {
            sum++;
        }
        if(sum>1)
        {
            break;
        }
    }
    if(n%2==1)
    {
        if(sum==1||sum==0)
        {
            printf("YES\n");
        }
        else printf("NO\n");
    }
    else
    {
        if(sum==1)
        {
            printf("YES\n");
        }
        else printf("NO\n");
    }
    return 0;
}

思路很简单，就是走一遍，看看前后有没有不一样的，不一样的就计数，因为只允许改动一个，所以当超过两个的时候就退出，在这个基础上进行奇数偶数的判断，如果是奇数的话当sum是0或者1的时候可以，其他情况不行，当是偶数的时候只有sum是1的时候才可以，因为必须要改动一个，0的时候当然就不对了