How to Fix Merge Two Sorted Lists Errors

In this tutorial, you'll learn about How to Fix Merge Two Sorted Lists Errors. We cover key concepts, practical examples, and best practices.

Fix merge two sorted lists errors when recursion overflows stack for long lists or dummy node fails.

Quick Fix

Wrong

def merge(l1,l2):
    if not l1: return l2
    if not l2: return l1
    if l1.val<l2.val:
        l1.next=merge(l1.next,l2); return l1
    else:
        l2.next=merge(l1,l2.next); return l2

Stack overflow for 10k+ nodes.

Right

def merge(l1,l2):
    d=c=ListNode(0)
    while l1 and l2:
        if l1.val<l2.val: c.next=l1; l1=l1.next
        else: c.next=l2; l2=l2.next
        c=c.next
    c.next=l1 or l2
    return d.next

1->2->4 + 1->3->4 -> 1->1->2->3->4->4. O(n+m).

Prevention

Use dummy head. Always advance curr. Append remaining at end.

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