hdu 3294 Girls' research

题意：求最长回文串

用Manacher 可以在O（n)时间内解决

#include<iostream>
#include<algorithm>
#include<string.h>
#define MAXN 200010
using namespace std;
char str1[MAXN],str[MAXN*2];
int rad[MAXN*2],nn,n;
void Manacher(int *rad,char *str,int n)
{
    int i;
    int mx = 0;
    int id;
    for(i=1; i<n; i++)
    {
        if( mx > i )
			rad[i] = rad[2*id-i]<mx-i?rad[2*id-i]:mx-i;        
        else
            rad[i] = 1;
        for(; str[i+rad[i]] == str[i-rad[i]]; rad[i]++)
            ;
        if( rad[i] + i > mx )
        {
            mx = rad[i] + i;
            id = i;
        }
    }
}
int main()
{
	char c;
	while(scanf("%c %s",&c,str1)==2)
	{
		getchar();
		int t=c-'a';
		nn=strlen(str1);
		for(int i=0;i<nn;i++)
		{
			int tt=str1[i]-'a';
			str1[i]=(tt-t+26)%26 + 'a';
		}
		n=2*nn+2;
		str[0]='$';
		for(int i=0;i<=nn;i++)
		{
			str[2*i+1]='#';
			str[2*i+2]=str1[i];
		}
		Manacher(rad,str,n);
		int ans=1,st;
		for(int i=0;i<n;i++)
		{
			if(rad[i]>ans)
			{
				ans=rad[i];
				st=i;
			}
		}
		ans--;
		st=st/2 + ans/2-1;
		if(ans==1)
			puts("No solution!");
		else
		{
			printf("%d %d\n",st+1-ans,st);
			for(int i=st+1-ans;i<=st;i++)
				printf("%c",str1[i]);
			puts("");
		}
	}
	return 0;
}