Manacher 之 模版

#include <bits/stdc++.h>

using namespace std;
#define ll long long
#define mem(a, b) memset(a,b,sizeof(a))
#define INF 0x3f3f3f3f
#define DBG printf("this is a input\n")
#define fi first
#define se second
#define mk(a, b) make_pair(a,b)
#define p_queue priority_queue

ll gcd(ll a, ll b) {
    return b == 0 ? a : gcd(b, a % b);
}

ll lcm(ll a, ll b) {
    return a / gcd(a, b) * b;
}
int d[31000000];
char s[11000005];
char str[31000000];
int ans = 0;
void Manacher()
{
    int len = strlen(str);
    int l = 0 , r = -1;
    for(int i = 0 ; i < len ; i ++)
    {
        int k = (i > r) ? 1 : min(r-i,d[r+l-i]);
        while(0 <= i - k && i + k < len && str[i-k] == str[i+k]) k ++;
        d[i] = k --;
        ans = max(ans,d[i]-1);
        if(i + k > r)
        {
            l = i - k;
            r = i + k;
        }
    }
}
int main(void)
{
    scanf("%s",s);
    int index = 0;
    int len = strlen(s);
    for(int i = 0 ; i < len ; i ++)
    {
        str[index ++] = '#';
        str[index ++] = s[i];
    }
    str[index ++] = '#';
    Manacher();
    printf("%d\n",ans);
}