学步园

#include <iostream>
#include <algorithm>
#include <cstdio>
#include <cstring>
using namespace std;

const int length = 110000 + 10;
char s[length * 2];
int p[length * 2];
 
int main(void)
{
	freopen("1.txt", "r", stdin);
	
	while(scanf("%s", s) != EOF)
	{
		int len = strlen(s), id = 0, mx = 0;
		for(int i = len; i >= 0; --i)
		{
			s[2 * i + 2] = s[i];
			s[2 * i + 1] = '#';
		}
		s[0] = '*';
		
		for(int i = 2; i < 2 * len + 1; ++i)
		{
			if(id + p[id] > i)
				p[i] = min(p[2 * id - i], id + p[id] - i);
			else
				p[i] = 1;
			while(s[i + p[i]] == s[i - p[i]])
				++p[i];
			if(i + p[i] > id + p[id])
				id = i;
			if(mx < p[i])
				mx = p[i];
		}
		cout << mx - 1 << endl;
	}
	
	return 0;
}

两个例子用于理解