GATE | GATE-CS-2017 (Set 1) | Question 30

Consider the C code fragment given below.

typedef struct node
{
    int data;
    node* next ;
} node;

void join(node* m, node* n)
{
    node* p = n;
    while (p->next != NULL)
    {
        p = p->next;
    }
    p–>next = m;
}

Assuming that m and n point to valid NULL- terminated linked lists, invocation of join will
(A) append list m to the end of list n for all inputs
(B) either cause a null pointer dereference or append list m to the end of list n
(C) cause a null pointer dereference for all inputs.
(D) append list n to the end of list m for all inputs.


Answer: (B)

Explanation: As it is stated in the question, that m and n are valid Lists but not explicitly specified if the lists are empty or not. We can have two cases:

  1. Case 1: If lists are not NULL : Invocation of join will append list m to the end of list n if the lists are not NULL. For Example:Before join operation :
    m =1->2->3->4->5->null
    n =6->7->8->9->nullAfter join operation :
    6->7->8->9->1->2->3->4->5->null
  2. Case 2: If lists are NULL : If the list n is empty and itself NULL, then joining and referencing would obviously create NULL pointer issue.

Therefore option B is correct

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