【manacher】模板

考试竟然写错了manacher！太耻辱了！所以赶快又敲了一遍模板！！一定不能错了aaaa

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

char a[11000005], M[22000010];
int max_r[22000010], ans;

void manacher ( ) {
    int len = strlen ( a );
    M[0] = '@'; M[1] = '#';
    for ( int i = 0; i < len; i ++ ) {
        M[i*2+2] = a[i];
        M[i*2+3] = '#';
    }
    M[len*2+2] = '$';
    int center = 0, mx = 0;
    for ( int i = 1; i <= len * 2 + 2; i ++ ) {
        if ( mx > i ) max_r[i] = min ( mx - i, max_r[center * 2 - i] );
        else max_r[i] = 1;
        while ( M[i + max_r[i]] == M[i - max_r[i]] ) max_r[i] ++;
        if ( mx < i + max_r[i] ) {
            mx = i + max_r[i]; center = i;
        }
        ans = max ( ans, max_r[i] - 1 );
    }
}

int main ( ) {
    scanf ( "%s", a );
    manacher ( );
    printf ( "%d", ans );
    return 0;
}