poj3461(KMP算法)

Oulipo

Time Limit: 1000MS		Memory Limit: 65536K
Total Submissions: 45875		Accepted: 18311

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

分析：求sub串在s串中出现的次数，kmp算法，
next[i]表示sub[0]到sub[i]子串中最大相同前后缀长度,并且next[0]=0。

#include<cstdio>
#include<cstring>
char s[1000010],sub[10010];
int next[10010],m,n;
void kmp()
{
    next[0]=0;
    for(int q=1,k=0;q<m;q++)
    {
        while(k>0&&sub[q]!=sub[k])
            k=next[k-1];//这步是重点，回溯  
        if(sub[q]==sub[k]) k++;
        next[q]=k;
    }
}
int main()
{
    int T;
    scanf("%d",&T);
    while(T--)
    {
        scanf("%s%s",sub,s);
        m=strlen(sub),n=strlen(s);
        kmp();
        int ans=0;
        for(int q=0,k=0;q<n;q++)//q是s的指针,k是sub的指针 
        {
            while(k>0&&sub[k]!=s[q])
                k=next[k-1];
            if(sub[k]==s[q]) k++;
            if(k==m) ans++;
        }
        printf("%d
",ans);
    }
    return 0;
}