kmp优化

https://vjudge.net/contest/70325?utm_source=qq&utm_medium=social#problem/B

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

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#include<iostream>  
#include<cstdio>  
#include<cstdlib>  
#include<cstring>  
#include<string>  
#include<queue>  
#include<algorithm>  
#include<map>  
#include<iomanip>  
#define INF 99999999  
using namespace std;  

const int MAX=1000000+10;  
char text[MAX];  
char word[10000+10];  
int next[10000+10];  

void get_next(char *a){//计算next,next会算到next[0...len]   
    int i=0,j=-1,len=strlen(a);  
    next[0]=-1;  
    while(i<len){  
        if(j == -1 || a[i] == a[j]){  
            if(a[++i] != a[++j])next[i]=j;  
            else next[i]=next[j];//一个优化,表示如果在kmp时a[i]和b[k]不匹配,那么a[i]=a[j],a[j]肯定和b[k]不匹配,所以用next[j]去匹配   
        }else j=next[j];  
    }  
}  

int KMP(char *a,char *b){  
    int i=0,j=0,lena=strlen(a),lenb=strlen(b),sum=0;  
    while(i<=lena && j<lenb){  
        if(i == -1 || a[i] == b[j])++i,++j;  
        else i=next[i];  
        if(i == lena)++sum;  
    }  
    return sum;  
}  

int main(){  
    int n;  
    cin>>n;  
    while(n--){  
        cin>>word>>text;  
        get_next(word);  
        cout<<KMP(word,text)<<endl;  
    }  
    return 0;  
}