hdu 3294 manacher 求回文串

#include <cstdio>
#include <cstring>
#include <iostream>
#define maxn 222222
using namespace std;

int n, ans;
char s[maxn];
int r[maxn];

int main() {
while( scanf( "%s", s )==1 ) {
n = strlen(s);
for( int i=n; i>=0; i-- ) {
s[(i<<1)+2] = s[i];
s[(i<<1)+1] = '#';
}
s[0] = '^';
s[n+n+2] = '$';
for( int i=2,id=1; i<=n+n+1; i++ ) {
if( id+r[id]>=i ) r[i] = min( r[id+id-i], id+r[id]-i );
else r[i] = 0;
while( s[i-r[i]-1]==s[i+r[i]+1] ) r[i]++;
if( id+r[id]<i+r[i] ) id=i;
}
ans = 0;
for( int i=1; i<=n+n+1; i++ )
ans = max( ans, r[i] );
printf( "%d\n", ans );
}
}