C程序--判断回文字符串

#include<stdio.h>
#include<string.h>
#define N 40
int main()
{
    int  i = 0,len,flag =0;
    char a[N];
    gets(a);
    len = strlen(a);
    for(i = 1 ; i<len-i-1;i++)
    {
          if(a[i] == a[len-i-1])
            flag = 1;
          if(a[i] != a[len-1-i])
            {flag = 0;
            break ;
            }
    }

    if(flag == 1)
      printf("YES\n");
    if(flag ==0)
      printf("NO\n");



    return 0 ;
}