字符串算法-KMP-Oulipo

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

Code

#include<iostream>
#include <string.h>
#include <string>
#include <stdio.h>
using namespace std;


const int MAXN=1e6+1;
int nxt[MAXN];
char w[MAXN],t[MAXN];

inline void get_next(){
    int p=nxt[0]=0;
    for(int i=1;i<strlen(w);i++){
        while(p&&w[p]!=w[i])    p=nxt[p-1];
        if(w[p]==w[i]) p++;
        nxt[i]=p;
    }
}

inline int kmp(){
    get_next();
    int ans=0;
    int p=0;
    for(int i=0;i<strlen(t);i++){
        while(p&&w[p]!=t[i]) p=nxt[p-1];
        if(w[p]==t[i]) p++;
        if(p==strlen(w)){
            ans++;
            p=nxt[p-1];
        }
    }
    return ans;
}

int main(){
    //freopen("/Users/guoyu/Desktop/algorithm/in.txt", "r", stdin);
    int T;cin>>T;
    while(T--){
        scanf("%s",w);scanf("%s",t);
        get_next();
        cout<<kmp()<<endl;
    }
    return 0;
}