Oulipo (KMP)

题目链接：https://vjudge.net/contest/248066#problem/B

参考链接：https://blog.youkuaiyun.com/qq_28175337/article/details/47336777

Oulipo

 

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

next[i]记录的是 i 之前的（不包括 i ）一列字符串的最大的最大的前后缀的位置;

举个栗子：

lens1  =  5

 0   1    2   3    4    5   6       下标

 A   B   C   A    B    D           字符串

-1   0   0    0    1    2    0       next 数组

如果与  A  B  C  A  B  A  B  C  A  B   D  比较

#include<cstdio>
#include<cstring>
#include<iostream>
using namespace std;
const int MAXN=1000005;
char s1[MAXN];
char s2[MAXN];
int next[MAXN];

int lens1,lens2;

void getnext(){

    int j=-1;
    int i=0;
    next[0]=-1;
    while(i<lens1){
        if(j==-1 || s1[i]==s1[j]){
            i++;
            j++;
            next[i]=j;
        }else{
            j=next[j];
        }
    }
}

int kmp(){
    int ans=0;
    int i=0;
    int j=0;
    while(i<lens2){
        cout<<i<<' '<<j<<endl;
        if(j==-1 ||s1[j]==s2[i]){
            i++;
            j++;
            if(j==lens1){
                ans++;
            }
        }else{
            j=next[j];
        }
    }
    return ans;
}

int main(){

    int t;
    cin>>t;
    while(t--){
        scanf("%s",s1);
        scanf("%s",s2);
        lens1=strlen(s1);
        lens2=strlen(s2);

        getnext();

//        for(int i=0;i<=lens1;i++){
//            cout<<next[i]<<' ';
//        }

        cout<<endl;
        cout<<kmp()<<endl;
    }
    return 0;
}