[ABC284F] ABCBAC（字符串哈希）

思路

这里我们要注意以下几点：

1. 字符串哈希自然溢出（$(\mathrm{mod}2{)}^{6}4$）会被卡，会$WA5$个点
2. 注意有模数的时候不要用$unsignedlonglong$类型

代码

#include <iostream>
using namespace std;
typedef long long LL;
const int N = 2000010,BASE = 131,MOD = 998244353;
int n;
char s[N];
LL p[N],h1[N],h2[N];
LL query1 (int l,int r) {
	return ((h1[r] - h1[l - 1] * p[r - l + 1]) % MOD + MOD) % MOD;
}
LL query2 (int l,int r) {
	return ((h2[l] - h2[r + 1] * p[r - l + 1]) % MOD + MOD) % MOD;
}
int main () {
	scanf ("%d%s",&n,s + 1);
	p[0] = 1;
	for (int i = 1;i <= 2 * n;i++) {
		p[i] = p[i - 1] * BASE % MOD;
		h1[i] = (h1[i - 1] * BASE + s[i] - 'a' + 1) % MOD;
	}
	for (int i = 2 * n;i >= 1;i--) h2[i] = (h2[i + 1] * BASE + s[i] - 'a' + 1) % MOD;
	for (int i = 1;i <= n + 1;i++) {
		if (query1 (1,i - 1) == query2 (n + 1,n + i - 1) && query1 (n + i,2 * n) == query2 (i,n)) {
			for (int j = n + i - 1;j >= i;j--) putchar (s[j]);
			printf ("\n%d",i - 1);
			return 0;
		}
	}
	puts ("-1");
	return 0;
}