学了这个算法,思路倒是不难懂,但是看了代码后却很绕,特别晕,一直有一个地方不太懂,现在才颇有感受。
next[0]为什么要赋值-1,以及这样的话char数组与next不就不对应了吗?
比如a[3]对应的next数组便是next[4],但是跳跃的时候却是用next[3]
直到后来我准备自己实施代码时发现上面的做法太棒了
举个例子
数组1 : a b c a b y x
对应的next数组:-1 0 0 0 1 2 0 0
数组2 :abcabcabyx
如图,当对应到第5个元素时(对应数组的下标),数组1为y,数组2为c,不匹配,于是第一个数组的下标j = next[j],即j = next[5] = 2,所以第一个数组下标就跳到第2个元素,也就是c了,c再与数组2的c匹配,完美!
如果不加上next[0] = -1会变成怎么样?那么每次匹配不成功,必须j = next[j-1],而且无法判断是否已经回到了首元素。
附上刚刷的一道例题:
Time Limit: 1000MS | Memory Limit: 65536K | |
Total Submissions: 46057 | Accepted: 18369 |
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
题意:
找出第一个字符串在第二个字符串中出现次数。
附上AC代码:
#include <cstdio> #include <iostream> #include <algorithm> #include <cmath> #include <cstdlib> #include <cstring> #include <vector> #include <list> #include <map> #include <stack> #include <queue> using namespace std; #define ll long long char a[11111]; int next[11111]; char b[1111111]; int lena; int lenb; int sum = 0; void getnext() { int i,j; next[0] = j = -1; i = 0; while(i < lena) { while(j != -1 && a[i] != a[j]) j = next[j]; next[++i] = ++j; } } void kmp() { int i,j; j = 0; i = 0; while(i < lenb) { while(j != -1 && a[j] != b[i]) j = next[j]; i++;j++; if(j >= lena) { sum++; j = next[j]; } } } int main() { int t; scanf("%d",&t); while(t--) { sum = 0; scanf("%s%s",a,b); lena = strlen(a); lenb = strlen(b); getnext(); kmp(); /*for(int i = 0;i <= len;i++) cout << next[i] << ' '; cout << endl;*/ printf("%d\n", sum); } return 0; }