KMP算法: Oulipo

这是一道北大OJ上的道

题目描述

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.

输入

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| ≤ 20,000 (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| ≤ 2,000,000.

输出

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.

样例输入

3
BAPC
BAPC
AZA
AZAZAZA
VERDI
AVERDXIVYERDIAN

样例输出

1
3
0

分析：

使用KMP算法匹配字符串

#include <bits/stdc++.h>using namespace std;string sam,t;
int nxt[1000050];
int sl,tl;void getnxt(){//求next数组int j = 0,k = -1;nxt[0] = -1;while(j < tl){if(k == -1||t[j] == t[k])nxt[++j] = ++k;else k = nxt[k];}
}int kmpcount(){//找总匹配数int res = 0;int i = 0;int j = 0;if(sl == 1&&tl == 1){if(sam[0] == t[0])return 1;else return 0;}getnxt();for(i = 0;i < sl;i++){while(j>0&&sam[i]!=t[j]){j = nxt[j];}if(sam[i] == t[j])j++;if(j == tl){res++;j = nxt[j];}}return res;
}int main(){int W;cin>>W;for(int l = 0;l<W;l++){cin>>t>>sam;sl = sam.size();tl = t.size();cout<<kmpcount()<<endl;}return 0;
}