POJ 3461 Oulipo KMP

最新推荐文章于 2020-07-22 02:59:48 发布

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原文链接：http://www.cnblogs.com/Loi-Vampire/p/6017057.html

本文介绍了一种用于字符串匹配的高效算法——KMP算法，并通过一个具体的编程实例详细展示了如何使用KMP算法来解决字符串匹配的问题。该算法可以快速找出一个单词在一段文本中的出现次数。

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

Source

BAPC 2006 Qualification

题目大意是这样的：
跟你多组数据，每组数据中包含两个字符串W和T，询问W在T中出现了多少次。

显然要用KMP

#include <iostream>
#include <cstdio>
#include <cstring>
using namespace  std;
const int SZ = 10010;
const int LA = 1000010;

int nxt[SZ];

void getnxt(char n[])
{
    nxt[0] = nxt[1] = 0;
    int len = strlen(n);
    for(int i = 1; i < len; i ++)
    {
        int j = nxt[i];
        while(j && n[i] != n[j])    j = nxt[j];
        nxt[i + 1] = (n[i] == n[j] ? j + 1 : 0);
    }
}

int kmp(char n[], char m[])
{
    int ans = 0;
    int len1 = strlen(n), len2 = strlen(m);
    for(int i = 0, j = 0; i < len2; i ++)
    {
        while(j && n[j] != m[i]) j = nxt[j];
        if(n[j] == m[i]) j ++;
        if(j == len1)   ans ++;
    }
    return ans;
}

int main()
{
    int t;
    scanf("%d", &t);
    while(t --)
    {
        memset(nxt, 0, sizeof(nxt));
        char n[SZ], m[LA];
        scanf("%s", n);
        scanf("%s", m);
        getnxt(n);
        printf("%d\n", kmp(n, m));
    }
}

转载于:https://www.cnblogs.com/Loi-Vampire/p/6017057.html