# [leetcode] 21. Merge Two Sorted Lists _ Algorithm Problem Solve for python

## 1. Problem

21. Merge Two Sorted Lists

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.

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 = 
Output: 


Constraints:

• 1 <= nums.length <= 5 * 10^5
• -2^31 <= nums[i] <= 2^31 - 1

## 2. Solution

I solved this problem like this.

It is easy to solve. We just compare list value and push next.

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next

class Solution:
def mergeTwoLists(self, list1: Optional[ListNode], list2: Optional[ListNode]) -> Optional[ListNode]:
ans = ListNode()
ans_end = ans
l1, l2 = list1, list2
while l1 and l2:
if l1.val < l2.val:
ans_end.next = l1
ans_end = l1
l1 = l1.next
else:
ans_end.next = l2
ans_end = l2
l2 = l2.next

while l1:
ans_end.next = l1
ans_end = l1
l1 = l1.next

while l2:
ans_end.next = l2
ans_end = l2
l2 = l2.next

return ans.next