HDU - 1686 Oulipo (KMP)

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.

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.

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.

3
BAPC
BAPC
AZA
AZAZAZA
VERDI
AVERDXIVYERDIAN

1
3
0
题意：给你一个匹配串，一个文本串，求匹配串在文本串出现的次数
思路：KMP的基本应用
#include <iostream>
#include <cstdio>
#include <cstring>
#include <algorithm>
using namespace std;
const int maxn = 1000010;

char a[maxn], b[maxn];
int next[maxn];
int n, m;

void getnext() {
	int m = strlen(a);
	next[0] = 0, next[1] = 0;
	for (int i = 1; i < m; i++) {
		int j = next[i];
		while (j && a[i] != a[j])
			j = next[j];
		next[i+1] = a[i] == a[j] ? j+1 : 0;
	}
}

int kmp() {
	int ans = 0;
	int n = strlen(b);
	int m = strlen(a);
	int j = 0;
	for (int i = 0; i < n; i++) {
		while (j && a[j] != b[i])
			j = next[j];
		if (a[j] == b[i])
			j++;
		if (j == m)
			ans++;
	}
	return ans;
}

int main() {
	int t;
	scanf("%d", &t);
	while (t--) {
		scanf("%s%s", a, b);
		getnext();
		int ans = kmp();
		printf("%d\n", ans);
	}
	return 0;
}