PAT Advanced Level 1040. Longest Symmetric String (25)

#include<cstdio>
#include<cstring>
#include<algorithm>
using namespace std;

char s[2000];
int ans,len,sum;

void check(int a,int b)
{
    if(s[a]!=s[b]) return;
    if(a==b) sum=1; else sum=2;
    int left=a-1,right=b+1;
    while(!(left<0||right>=len||s[left]!=s[right]))
        left--,right++,sum=sum+2;
    ans=max(ans,sum);
}

int main()
{
    gets(s); len=strlen(s); ans=0;
    for(int i=0;i<len;i++) check(i,i),check(i,i+1);
    printf("%d\n",ans);
    return 0;
}