21 Merge Two Sorted Lists

https://leetcode.com/problems/merge-two-sorted-lists/

Recursive §

Iterative §

• new node pointer is needed;
• We need a loop for (list1 && list2), and conditions for when list1’s value is larger than list2’s value, and vice versa
• After exiting the loop, check which list pointer is still valid, then simply append walker->next to that list pointer
• Remember to assign walker to head, then delete the head

Mistakes §

ListNode *head = new ListNode(); 
ListNode *walker = head; 
walker = list1; 
return head;

• walker is first assigned to the newly allocated ListNode object. However it is then assigned to list1; now there is nobody connected to head!
• Instead, do walker->next=list1 or head->next = walker