CODEWITHSUNDEEP
c++singly-linked-list
Eugene Wolf
Eugene Wolf

Reputation: 69

Struggling to convert an integer into a linked list. Not sure what I'm doing wrong here

The following code is supposed to take an integer (which is numerator) and convert it into a linked list, starting from the very end. As an example, for the integer 603, I want to create 3->0->6. But for some reason, my output only gives me 0->6, and completely ignores the 3? I've looked over my code and I just can't seem to see where my logic is going wrong. 

    struct ListNode {
      int val;
      ListNode *next;
      ListNode(int x) : val(x), next(NULL) {}
    }

    int numerator = sum1 + sum2;

    ListNode * ptr;

    while (numerator != 0) {

        int val = numerator % 10;
        numerator = numerator / 10;

        ListNode * node = new ListNode(val);

        // If list is empty, add node.
        if (!ptr) {
            ptr = node;
        // If list is not empty, traverse to end of list and then append.
        } else {

            while (ptr->next) {
                ptr = ptr->next;
            }
            ptr->next = node;
        }
   }

Upvotes: 0

Views: 168

Answers (1)

Maxim Egorushkin
Maxim Egorushkin

Reputation: 136208

Your code loses the head of the list because it is stored in ptr which is changed on each insertion.

You can append a node to a singly-linked list in a more efficient manner:

ListNode* head = 0;
ListNode** tail = &head;

// ...    

// Append the new node to the tail of the list.
ListNode* node = new ListNode(val);
*tail = node;
tail = &node->next;

Upvotes: 2

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