KMP improvement (nextval)

Why should we improve next? Because next has a defect... O (& cap; _ & cap;) O haha ~...

What are the defects?

The example in the instructor's PPT in the class is not very good and does not reflect the core. It is only a part of it. Therefore, please take a closer look at the courseware and the example below.

Set S string abcabdabcabcd; P string abcabcd

Okay, first find the next [] of the P string. (obedient, find next []. Don't look at it. Let's ask you to find next on your computer. No, code in the previous blog (http://www.cnblogs.com/xiaoshanshan/p/4081693.html ));

 

Well, you're probably not obedient. Forget it. Let's take a look.

Next [0 .. 6] = {-, 3 };

Next we will focus on it. Let's take a look at how KMP is implemented (KMP calls you, come out) (slag )... O (& cap; _ & cap;) O haha ~...

This process is a bit long. Please take a look at it. (it takes a longer time for me to launch ....)

S a (bcabdabcabcd) AB (cabdabcabcd) abc (abdabcabcd) abca (bdabcabcd) abcab (dabcabcd) abcabd (abcabcd)

P a (bcabcd) AB (cabcd) abc (abcd) abca (bcd) abcab (cd) abcabc (d)

(I wipe... Look at my next...Next [5] = 2)

S abcabd (abcabcd)

P abc (abcd)

(KMP, you can stop .) (Slag ).... O (& cap; _ & cap;) O haha ~...

 

You can see it. What are you looking? What do you mean by reading the above? Let's look down:

Just now P [next [5] = P [5] = 'C', right. Okay, now I understand. This match is white match, because it certainly does not match. Now you understand.

Nana? You still don't understand, P [5] = P [2], right, P [5] is not good, P [2] is definitely not good. (Next, call nextval)

 

So before I talk about nextval, let's take a look at how nextval works.

S a (bcabdabcabcd) AB (cabdabcabcd) abc (abdabcabcd) abca (bdabcabcd) abcab (dabcabcd) abcabd (abcabcd)

P a (bcabcd) AB (cabcd) abc (abcd) abca (bcd) abcab (cd) abcabc (d)

(Ah !! Check my nextval ...)

 

S abcabd (abcabcd)

P a (bcabcd)(Key part)

(Nana? I will try again ...)

 

S abcabda (bcabcd)

P a (bcabcd) (I will not write it next, and finally match it)

 

Here, please pay attention to (key part). How is it? Is nextval a little better than next (please refer to their differences ).

Okay, I don't want to talk much about it. Let's go to the code.

1 void Get_next (string P, int * nextval) {2 int I, j; 3 nextval [I = 0] = j =-1; 4 int len = P. length (); 5 while (I <len-1) {6 if (j =-1 | P [I] = P [j]) {7 I ++, j ++; 8 if (P [I] = P [j]) {9 nextval [I] = nextval [j]; 10} 11 else nextval [I] = j; 12} 13 else j = nextval [j]; 14} 15}

 

It's so simple.

Exercise, what is the nextval of the P string abcabcd?

Nextval [0 .. 7] = {-, 0 };

 

KMP improvement (nextval)