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;
}