时间复杂度为O(n),参考:http://bbs.dlut.edu.cn/bbstcon.php?board=Competition&gid=23474
#include<iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
#define maxn 2001000
using namespace std;
char s[maxn],st[maxn];
int p[maxn];
int cases=1;
int n;
void rebuild()
{
st[0]='$';
st[1]='#';
int len=strlen(s);
for(int i=0;i<len;i++)
{st[2*i+2]=s[i];st[2*i+3]='#';}
st[2*len+2]=0;
}
void solve()
{
int len=2*strlen(s)+2;
int id,mx=0,ans=1;
for(int i=0;i<len;i++)
{
if(mx>i)
p[i]=min(p[2*id-i],mx-i);
else
p[i]=1;
for(;st[p[i]+i]==st[i-p[i]];p[i]++)
;
if(p[i]+i>mx)
{
mx=p[i]+i;
id=i;
}
ans=max(ans,p[i]);
}
cout<<"Case "<<cases++<<": "<<ans-1<<endl;
}
int main()
{
while(gets(s))
{
if(s[0]=='E'&&s[1]=='N'&&s[2]=='D') break;
rebuild();
solve();
}
return 0;
}