Hotaru's problem
Time Limit: 4000/2000 MS (Java/Others) Memory Limit: 65536/65536 K (Java/Others)
Total Submission(s): 3314 Accepted Submission(s): 1101
Problem Description
Hotaru Ichijou recently is addicated to math problems. Now she is playing with N-sequence.
Let's define N-sequence, which is composed with three parts and satisfied with the following condition:
1. the first part is the same as the thrid part,
2. the first part and the second part are symmetrical.
for example, the sequence 2,3,4,4,3,2,2,3,4 is a N-sequence, which the first part 2,3,4 is the same as the thrid part 2,3,4, the first part 2,3,4 and the second part 4,3,2 are symmetrical.
Let's define N-sequence, which is composed with three parts and satisfied with the following condition:
1. the first part is the same as the thrid part,
2. the first part and the second part are symmetrical.
for example, the sequence 2,3,4,4,3,2,2,3,4 is a N-sequence, which the first part 2,3,4 is the same as the thrid part 2,3,4, the first part 2,3,4 and the second part 4,3,2 are symmetrical.
Give you n positive intergers, your task is to find the largest continuous sub-sequence, which is N-sequence.
Input
There are multiple test cases. The first line of input contains an integer T（T<=20）, indicating the number of test cases.
For each test case:
the first line of input contains a positive integer N（1<=N<=100000）, the length of a given sequence
the second line includes N non-negative integers ,each interger is no larger than 109 , descripting a sequence.
Output
Each case contains only one line. Each line should start with “Case #i: ”,with i implying the case number, followed by a integer, the largest length of N-sequence.
We guarantee that the sum of all answers is less than 800000.
Sample Input
1
10
2 3 4 4 3 2 2 3 4 4
10
2 3 4 4 3 2 2 3 4 4
Sample Output
Case #1: 9
Author
UESTC
Source
//在manachar的基础上，枚举回文串的中心，再找第三部分。
#include<cstdio>
#include<cstring>
#include<iostream>
using namespace std;
int read(){
register int x=;bool f=;
register char ch=getchar();
while(ch<''||ch>''){if(ch=='-')f=;ch=getchar();}
while(ch>=''&&ch<=''){x=x*+ch-'';ch=getchar();}
return f?x:-x;
}
const int N=3e5+;
int n,ans,cas,l,T,s[N],S[N],p[N];
void manacher(){
int id=,mx=-;
for(int i=;i<l;i++){
if(id+mx>i) p[i]=min(p[id*-i],id+mx-i);
while(i-p[i]>=&&i+p[i]<=l&&S[i-p[i]]==S[i+p[i]]) p[i]++;
if(id+mx<i+p[i]) id=i,mx=p[i];
}
}
void init(){
l=;memset(p,,sizeof p);
for(int i=;i<n;i++) S[++l]=-,S[++l]=s[i];
S[++l]=-;
}
int main(){
for(T=read(),cas=;ans=,cas<=T;cas++){
n=read();
for(int i=;i<n;i++) s[i]=read();
init();manacher();
for(int i=;i<=n*+;i+=){
for(int j=i+p[i]-;j-i>ans;j-=){
if(j-i+<=p[j]){
ans=max(ans,j-i);
break;
}
}
}
printf("Case #%d: %d\n",cas,ans/*);
}
return ;
}