E - Oulipo HDU - 1686 (kmp算法)

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本文介绍了一个基于KMP算法的比赛题目，任务是在一个长文本中快速查找特定字符串出现的次数，包括了KMP算法的实现代码。

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E - Oulipo

HDU - 1686

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

直接套用kmp算法，统计匹配的个数即可

code:

#include <iostream>
#include <cstdio>
#include <cstring>
using namespace std;
int Next[100000];
char w[100000],t[10000000];
void getNext(){//求next数组的函数
    int i = -1,j = 0;//两个下标错开
    Next[0] = -1;//初始化next[0]=-1
    int lenw = strlen(w);
    while(j < lenw){
        if(i==-1||w[i]==w[j]){
            i++,j++;//相等或i=-1，ij同时加
            if(w[i]==w[j])Next[j] = Next[i];//这一步是个优化，如果相同等于这个相同这个点i的next[i]
            else Next[j] = i;//不同直接等于i
        }
        else i = Next[i];//不同从next[i]开始匹配
    }
    return;
}
int Kmp(){
    int cnt = 0;
    int i = 0,j = 0;
    int lenw = strlen(w);
    int lent = strlen(t);
    getNext();//得到next数组
    while(j < lent){
        if(i==-1||w[i]==t[j]){
            i++,j++;//类似于getnext函数
            if(i==lenw){
                cnt++;//如果模式字符串到了末尾个数++，可以不用对i进行操作i会通过next数组自己初始回去
            }
        }
        else i = Next[i];
    }
    return cnt;
}
int main(){
    int num;
    scanf("%d",&num);
    while(num--){
        scanf("%s%s",w,t);
        printf("%d\n",Kmp());
    }
    return 0;
}