KMP算法

Problem Description
The French author Georges Perec (1936–1982) once wrote a book, La disparition, without the letter 'e'. He was a member of the Oulipo group. A quote from the book:

Tout avait Pair normal, mais tout s’affirmait faux. Tout avait Fair normal, d’abord, puis surgissait l’inhumain, l’affolant. Il aurait voulu savoir où s’articulait l’association qui l’unissait au roman : stir son tapis, assaillant à tout instant son imagination, l’intuition d’un tabou, la vision d’un mal obscur, d’un quoi vacant, d’un non-dit : la vision, l’avision d’un oubli commandant tout, où s’abolissait la raison : tout avait l’air normal mais…

Perec would probably have scored high (or rather, low) in the following contest. People are asked to write a perhaps even meaningful text on some subject with as few occurrences of a given “word” as possible. Our task is to provide the jury with a program that counts these occurrences, in order to obtain a ranking of the competitors. These competitors often write very long texts with nonsense meaning; a sequence of 500,000 consecutive 'T's is not unusual. And they never use spaces.

So we want to quickly find out how often a word, i.e., a given string, occurs in a text. More formally: given the alphabet {'A', 'B', 'C', …, 'Z'} and two finite strings over that alphabet, a word W and a text T, count the number of occurrences of W in T. All the consecutive characters of W must exactly match consecutive characters of T. Occurrences may overlap.

Input
The first line of the input file contains a single number: the number of test cases to follow. Each test case has the following format:

One line with the word W, a string over {'A', 'B', 'C', …, 'Z'}, with 1 ≤ |W| ≤ 10,000 (here |W| denotes the length of the string W).
One line with the text T, a string over {'A', 'B', 'C', …, 'Z'}, with |W| ≤ |T| ≤ 1,000,000.
Output
For every test case in the input file, the output should contain a single number, on a single line: the number of occurrences of the word W in the text T.

Sample Input

3
BAPC
BAPC
AZA
AZAZAZA
VERDI
AVERDXIVYERDIAN

Sample Output

1
3
0

 

这是一道考KMP的算法题，需要注意的是当匹配了一个模式串P后，如何选择i和j。

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <string>
#include <iostream>

using namespace std;

char T[1000010], P[10010];
int  f[10010];

void KMPFailureFunction()
{
	int i, j;
	int m = strlen(P);

	i = 1;
	j = 0;
	while(i < m)
	{
		if(P[j] == P[i])
		{
			f[i] = j + 1;
			++i;
			++j;
		} else if(j > 0)
			j = f[j - 1];
		else {
			f[i] = 0;
			++i;		
		}
	}
}

int KMP()
{
	KMPFailureFunction();

	int ans = 0;
	int i, j;
	int n = strlen(T);	
	int m = strlen(P);

	i = 0;
	j = 0;
	while(i < n)
	{
		if(P[j] == T[i])
		{
			if(j == m-1)
			{
				ans++;
				++i;
				j = f[j];

			} else {
				++i;
				++j;
			}
		} else if(j > 0)
			j = f[j - 1];
		else ++i;
	}

	return ans;
}


int main()
{
	//freopen("data_B.txt", "r", stdin);
	int t;
	scanf("%d", &t);
	
	while(t--)
	{
		scanf("%s%s", P, T);
		printf("%d\n", KMP());
	}
	
	return 0;
}