HDU 3068 最长回文(Manacher算法解决最长回文串问题)

这算是个历史遗留问题了啊，当时没怎么看，最近又发现了，标记一下。脑子越来越不好使了啊。

Manacher算法：http://blog.csdn.net/xingyeyongheng/article/details/9310555

解释一下：p[id]+id表示最远(从左向右)可以到达的字符串。

最长回文

Time Limit: 4000/2000 MS (Java/Others)    Memory Limit: 32768/32768 K (Java/Others)

Total Submission(s): 18167    Accepted Submission(s): 6677


Problem Description

给出一个只由小写英文字符a,b,c...y,z组成的字符串S,求S中最长回文串的长度.

回文就是正反读都是一样的字符串,如aba, abba等

 

Input

输入有多组case,不超过120组,每组输入为一行小写英文字符a,b,c...y,z组成的字符串S

两组case之间由空行隔开(该空行不用处理)

字符串长度len <= 110000

 

Output

每一行一个整数x,对应一组case,表示该组case的字符串中所包含的最长回文长度.

 

Sample Input

aaaa

abab

 

Sample Output

4
3

 

Source

2009 Multi-University Training Contest 16 - Host
by NIT

 

#include <algorithm>
#include <iostream>
#include <stdlib.h>
#include <string.h>
#include <iomanip>
#include <stdio.h>
#include <string>
#include <queue>
#include <cmath>
#include <stack>
#include <ctime>
#include <map>
#include <set>
#define eps 1e-9
///#define M 1000100
///#define LL __int64
#define LL long long
///#define INF 0x7ffffff
#define INF 0x3f3f3f3f
#define PI 3.1415926535898
#define zero(x) ((fabs(x)<eps)?0:x)
#define mod 1000000007

#define Read() freopen("autocomplete.in","r",stdin)
#define Write() freopen("autocomplete.out","w",stdout)
#define Cin() ios::sync_with_stdio(false)

using namespace std;

const int maxn = 222000;
char str[maxn];
char s[maxn];
int p[maxn];
int main()
{
while(~scanf("%s", s))
{
int len = strlen(s);
str[0] = '#';
str[1] = '$';
for(int i = 0, j = 2; i < len; i++)
{
str[j++] = s[i];
str[j++] = '$';
}
int id = 0;
int Max = 0;
memset(p, 0, sizeof(p));
for(int i = 2; i < 2*len+1; i++)
{
if(p[id]+id > i)
p[i] = min(p[2*id-i], p[id]+id-i);
else
p[i] = 1;
while(str[i-p[i]] == str[i+p[i]]) p[i]++;
if(p[id]+id < i+p[i]) id = i;
Max = max(Max, p[i]);
}
cout<<Max-1<<endl;
}
return 0;
}