Here is the link to the problem: Merge Two Sorted List.

Problem Statement:

You are given the heads of two sorted linked lists list1 and list2.

Merge the two lists into one sorted list. The list should be made by splicing together the nodes of the first two lists.

Return the head of the merged linked list.

Examples:

Example 1:

Input: list1 = [1,2,4], list2 = [1,3,4]

Output: [1,1,2,3,4,4]

Example 2:

Input: list1 = [], list2 = []

Output: []

Example 3:

Input: list1 = [], list2 = [0]

**Output: [0]

My Solution

# Definition for singly-linked list.
# class ListNode(object):
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution(object):
    def mergeTwoLists(self, list1, list2):
        """
        :type list1: Optional[ListNode]
        :type list2: Optional[ListNode]
        :rtype: Optional[ListNode]
        """
        a = 0
        b = 0
        new_list = []
        while a < len(list1) and b < len(list2):
            if list1[a] < list2[b]:
                new_list.append(list1[a])
                a += 1
            elif list1[a] > list2[b]:
                new_list.append(list2[b])
                b += 1
            else:
                new_list.append(list1[a])
                new_list.append(list1[a])
                a += 1
                b += 1

        while a != len(list1):
            new_list.append(list1[a])
            a += 1

        while b != len(list2):
            new_list.append(list2[b])
            b += 1

Solution with linked list:

# Definition for singly-linked list.
# class ListNode(object):
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution(object):
    def mergeTwoLists(self, list1, list2):
        """
        :type list1: Optional[ListNode]
        :type list2: Optional[ListNode]
        :rtype: Optional[ListNode]
        """
        dummy = ListNode()
        tail = dummy

        while list1 and list2:
            if list1.val < list2.val:
                tail.next = list1
                list1 = list1.next
            else:
                tail.next = list2
                list2 = list2.next
            
            tail = tail.next
            
        if list1:
            tail.next = list1
        elif list2:
            tail.next = list2
        
        return dummy.next