回文的概念：顺读和倒读是一样的

比如：12321

           madam

           上海自来水来自海上  

1.法一

#include<stdio.h>
#include<string.h>
int main()
{char s[100] ;gets(s);int start = 0, end = strlen(s) - 1;int flag = 1;while (start <= end && 1 == flag){flag = (s[start] == s[end]);start++;end--;}if (1 == flag)printf("%s是回文",s);else printf("%s不是回文",s);return 0;
}

2.法二

#include<stdio.h>
#include<string.h>
int main()
{char s[100] ;gets(s);int start = 0, end = strlen(s) - 1;int flag = 1;while (start <= end && 1 == flag){flag = (s[start++] == s[end--]);}1 == flag ? printf("%s是回文", s) : printf("%s不是回文", s);return 0;
}

3.法三

#include<stdio.h>
#include<string.h>
int main()
{char s[100] ;gets(s);int start = 0, end = strlen(s) - 1;int flag = 1;while (start <= end && flag == (s[start++] == s[end--]));1 == flag ? printf("%s是回文", s) : printf("%s不是回文", s);return 0;
}