A null pointer exception to note about the list algorithm

In the study of data structure, I believe that many pen pals will be pointers to this knowledge point trapped, a small design to the pointer algorithm may take you a lot of time to be able to perfect, to blame the end is not aware of the occurrence of null pointers. I'm holding a chestnut below:

A single linked list L with a leading node is known, and its nodes are defined as follows:

1 template <typename t>2struct  linklist{3T data; 4 linklist<t> * head; 5 }

Now design an algorithm to find the logical structure of the last node with the value e:
Oh, look very simple, just need to go through the list to find the results, the following is attached to my original code:

1Template <typename t>2 intFindLast (linklistclass<t>l,t e)3 {4Linklist<t> * p=L.head;5     intposition=0, i=0;6     While (p!=null) ①7     {8       p=p->next; ②9i++;Ten        if (p->data==e) ③ Oneposition=i; A     } -      -     returnPosition//returns 0 if the lookup successfully returns its logical sequence number the}

Then I compile, link, there is no error, but during the operation of the error can not continue to run, this is a run-time error

See, do not know what is wrong, so I debug a bit, found that the root of the problem lies in the above source code in the order of 123 marks.

This function ignores the null pointer exception error, if the pointer p to the tail node of the chain list L, at this time into the while loop, after the statement ②,p==null, and then execute ③ will be packet null pointer exception

The problem is found, the solution will be much easier, below I cite a solution code (in fact there are many)

1Template<typename t>2 intFindLast (linklistclass<t>&L, T e)3 {4     intPosition =0;5     inti =1;6linklist<t> * p = l.head->Next;7      while(P! =NULL)8     {9         if(P->data = =e)TenPosition =i; Onei++; Ap = p->Next; -     } -     returnposition; the}

How, the problem is small, but need to pay attention to AH! Usually programming must pay attention to these boundary value problems, especially the algorithm that involves pointers.

A null pointer exception to note about the list algorithm