Leetcode: Merge Sorted Lists

Merge Two Sorted Lists | LeetCode OJ

We need to merge 2 sorted linked lists using constant additional space. We can follow the steps listed below

1. Initialize a dummy head
2. While both lists are non-empty attach the node with smaller value to the result
3. When one of the lists become empty attach the head of non-empty list to the result
4. Return the 2nd node of the result as the first node was dummy

Remarks:

1. O(1) space complexity as only 1 additional variable is used
2. O(min(m.n)) time complexity where m and n are the lengths of the given list. This is because we only iterate until both lists are non-empty
3. Used dummy head to avoid writing checks for empty input

Extensions:

1. Merge k sorted lists

# Definition for singly-linked list.
class ListNode(object):
    def __init__(self, x):
        self.val = x
        self.next = None
class Solution(object):
    def mergeTwoLists(self, l1, l2):
        """
        :type l1: ListNode
        :type l2: ListNode
        :rtype: ListNode
        """
        res = ListNode(0)
        iter = res
        while l1 and l2:
            if l1.val < l2.val:
                iter.next = l1
                l1 = l1.next
            else:
                iter.next = l2
                l2 = l2.next
            iter = iter.next
        if l1:
            iter.next = l1
        if l2:
            iter.next = l2
        return res.next