loj2073 「JSOI2016」扭动的回文串

ref
主要是要理解“撑到“最长这个概念
（为啥我的代码这么长QAQ

#include <iostream>
#include <cstdio>
using namespace std;
typedef unsigned long long ull;
int n, pa[200005], pb[200005], ans;
ull bse1[200005], bse2[200005], hsa1[200005], hsa2[200005], hsb1[200005];
ull hsb2[200005];
const int mod1=19260817, mod2=1e9+7;
char sa[200005], sb[200005];
bool iseq(int a, int b, int c, int d){
	if(a>b)	return true;
	ull sa1=0, sb1=0;
	if(a)
		sa1 = hsa1[a-1] * bse1[b-a+1] % mod1;
	sa1 = (hsa1[b] - sa1 + mod1) % mod1;
	if(d!=n-1)
		sb1 = hsb1[d+1] * bse1[d-c+1] % mod1;
	sb1 = (hsb1[c] - sb1 + mod1) % mod1;
	if(sa1!=sb1)	return false;
	sa1 = sb1 = 0;
	if(a)
		sa1 = hsa2[a-1] * bse2[b-a+1] % mod2;
	sa1 = (hsa2[b] - sa1 + mod2) % mod2;
	if(d!=n-1)
		sb1 = hsb2[d+1] * bse2[d-c+1] % mod2;
	sb1 = (hsb2[c] - sb1 + mod2) % mod2;
	if(sa1!=sb1)	return false;
	return true;
}
void manacher(char a[], int b[]){
	int id=0, mx=0;
	for(int i=1; i<n; i++){
		if(i<mx)	b[i] = min(b[2*id-i], mx-i);
		else	b[i] = 1;
		while(a[i-b[i]]==a[i+b[i]])	b[i]++;
		if(mx<i+b[i])	mx = i + b[i], id = i;
		ans = max(ans, b[i]);
	}
}
int main(){
	cin>>n;
	scanf("%s", sa);
	scanf("%s", sb);
	for(int i=n; i>=0; i--){
		sa[2*i+1] = '#';
		sa[2*i+2] = sa[i];
	}
	for(int i=n; i>=0; i--){
		sb[2*i+1] = '#';
		sb[2*i+2] = sb[i];
	}
	sa[0] = sb[0] = '$';
	n = 2 * (n + 1);
	bse1[0] = bse2[0] = 1;
	for(int i=1; i<n; i++){
		bse1[i] = bse1[i-1] * 131 % mod1;
		bse2[i] = bse2[i-1] * 131 % mod2;
	}
	ull ff=0;
	for(int i=0; i<n; i++){
		ff = (ff * 131 % mod1 + sa[i]) % mod1;
		hsa1[i] = ff;
	}
	ff = 0;
	for(int i=0; i<n; i++){
		ff = (ff * 131 % mod2 + sa[i]) % mod2;
		hsa2[i] = ff;
	}
	ff = 0;
	for(int i=n-1; i>=0; i--){
		ff = (ff * 131 % mod1 + sb[i]) % mod1;
		hsb1[i] = ff;
	}
	ff = 0;
	for(int i=n-1; i>=0; i--){
		ff = (ff * 131 % mod2 + sb[i]) % mod2;
		hsb2[i] = ff;
	}
	manacher(sa, pa);
	manacher(sb, pb);
	ans--;
	for(int i=1; i<n; i++){
		int pos1=i-pa[i], pos2=i+pa[i]-2;
		int l=0, r=min(pos1, n-pos2), mid, re;
		while(l<=r){
			mid = (l + r) >> 1;
			if(iseq(pos1-mid+1, pos1, pos2, pos2+mid-1))
				re = mid, l = mid + 1;
			else	r = mid - 1;
		}
		ans = max(ans, pa[i]+re-1);
	}
	for(int i=1; i<n; i++){
		int pos1=i-pb[i]+2, pos2=i+pb[i];
		int l=0, r=min(pos1, n-pos2), mid, re;
		while(l<=r){
			mid = (l + r) >> 1;
			if(iseq(pos1-mid+1, pos1, pos2, pos2+mid-1))
				re = mid, l = mid + 1;
			else	r = mid - 1;
		}
		ans = max(ans, pb[i]+re-1);
	}
	cout<<ans<<endl;
	return 0;
}