pku2406 Power Strings（kmp字符匹配,next数组）

   重看了下数据结构，发现有些知识还没有吃透。kmp算法就是一个。kmp的关键就是next数组，求出了next数组后就和O(n*m)的算法类似。求next数组有dp的思想，假设知道next[i]=k,现在要求next[i+1],
                next[i]+1   ch[i]=ch[i+1]
   next[i+1]=   next[k']+1  沿着前面的next数组找到一个k',ch[k']=ch[i+1],简单情况就是k'=next[i]
                0           其他情况
书上还说的了next数组的优化，当ch[k]==ch[i+1]时，next[i+1]=next[k]。
附段伪代码:

algorithm kmp_search:
    input:
        an array of characters, S (the text to be searched)
        an array of characters, W (the word sought)
    output:
        an integer (the zero-based position in S at which W is found)

    define variables:
        an integer, m ← 0 (the beginning of the current match in S)
        an integer, i ← 0 (the position of the current character in W)
        an array of integers, T (the table, computed elsewhere)

    while m + i is less than the length of S, do:
        if W[i] = S[m + i],
            let i ← i + 1
            if i equals the length of W,
                return m
        otherwise,
            let m ← m + i - T[i],
            if i is greater than 0,
                let i ← T[i]

    (if we reach here, we have searched all of S unsuccessfully)
    return the length of S

1. #include <iostream>
2. using namespace std;
3. #include <string.h>
4. #define pr printf
5.  char ch[1003003];
6. int next[1003003];
7. int main(int argc, char** argv) {
8.     int len,i;
9.     while(scanf("%s",ch))
10.     {
11.         len=strlen(ch);///书上的算法，数据结构
12.         if(len==1 && ch[0]=='.')break;
13.         next[1]=0;int j=0;i=1;
14.         while(i<len)
15.         {
16.             if(j==0 ||ch[i]==ch[j]){++i,++j;next[i]=j;}
17.             else j=next[j];
18.         }
19.         int tmp,cur,l; 
20.             l=len-next[len];
21.             tmp=len/(len-next[len]);
22.         if(len%l!=0){pr("1/n");continue;}
23.             else {
24.                  cur=l-1;
25.                  for(cur;cur<len;cur+=l)if(ch[cur]!=ch[l-1])break;
26.                 if(cur>=len){
27.                     pr("%d/n",tmp);
28.                 }else pr("1/n");
29.             }
30.     }
31.     return (EXIT_SUCCESS);
32. }