POJ3461题解——KMP入门
题目链接:http://poj.org/problem?id=3461
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
题目意思:就是先给你一个整数t表示有多少组样例,每组样例由两个字符串组成,找出第一个字符串在第二个字符串中出现的次数。
题目思路:这不就是kmp算法的模板题嘛。只不过我们按照kmp的规则去匹配当匹配完成以后(j==plen时)我们要做的不是返回模式串在文本串的位置,而是count++计数,并且继续往后找有没有再次和模式串匹配相同的,这里我们让j=next[j],而这也恰好是失配时的做法。
ac代码如下:
![](https://images.cnblogs.com/OutliningIndicators/ContractedBlock.gif)
#include<iostream> #include<stdio.h> #include<string.h> using namespace std; const int maxn=1e6+10; int n,count,next[maxn]; char s[maxn],p[maxn]; void GetNext(char *p) { int j=-1; int i=0; next[0]=-1; int plen=strlen(p); while(i<plen) { if(j==-1||p[i]==p[j]) { ++i;++j; if(p[i]==p[j])next[i]=next[j]; else next[i]=j; } else j=next[j]; } } void kmp(char *p,char *s) { int plen=strlen(p); int slen=strlen(s); int i=0; int j=0; while(i<slen&&j<plen) { if(j==-1||s[i]==p[j]) { ++i;++j; } else j=next[j]; if(j==plen) { count++; j=next[j]; } //printf("i=%d j=%d\n",i,j); } return; } int main() { scanf("%d",&n); while(n--) { scanf("%s%s",p,s); count=0; GetNext(p); /* for(int i=0;i<strlen(p);i++) printf("%d ",next[i]);printf("\n"); */ kmp(p,s);//printf("666\n"); printf("%d\n",count); } return 0; }