> ## Documentation Index
> Fetch the complete documentation index at: https://leetcode-py.wisl.dev/llms.txt
> Use this file to discover all available pages before exploring further.

> ## Agent Instructions
> leetcode-py is a Python LeetCode practice environment generator with one CLI: lcpy. It is not a service or platform.
> Each problem is a directory under leetcode/ with README.md, solution.py, test_solution.py, helpers.py, and playground.ipynb. lcpy gen creates them from JSON templates bundled with the package.
> Examples are backed by tests; copy them verbatim.

# Merge Two Sorted Lists Python Solution

> Tested Python solution for LeetCode 21 with 14 pytest cases. Generate a practice environment with lcpy.

LeetCode 21, Easy. Topics: Linked List, Recursion. [View on LeetCode](https://leetcode.com/problems/merge-two-sorted-lists/description/).

Generate this problem as a practice environment: tested reference solution, 14 [parametrized pytest cases](/practice/testing), and a playground notebook:

```bash theme={"theme":{"light":"github-light","dark":"github-dark"}}
lcpy gen -n 21   # by problem number
lcpy gen -s merge_two_sorted_lists   # by problem name
```

## Problem

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](https://assets.leetcode.com/uploads/2020/10/03/merge_ex1.jpg)

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

```
Input: list1 = [], list2 = []
Output: []
```

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

### Constraints

* The number of nodes in both lists is in the range `[0, 50]`.
* `-100 <= Node.val <= 100`
* Both `list1` and `list2` are sorted in **non-decreasing** order.

## Solution

Reference implementation from [solution.py on GitHub](https://github.com/wislertt/leetcode-py/blob/main/leetcode/merge_two_sorted_lists/solution.py), full suite in [test\_solution.py](https://github.com/wislertt/leetcode-py/blob/main/leetcode/merge_two_sorted_lists/test_solution.py):

```python theme={"theme":{"light":"github-light","dark":"github-dark"}}
from leetcode_py import ListNode


class Solution:
    # Time: O(m + n)
    # Space: O(1)
    def merge_two_lists(
        self, list1: ListNode[int] | None, list2: ListNode[int] | None
    ) -> ListNode[int] | None:
        dummy = ListNode(0)
        current = dummy

        while list1 and list2:
            if list1.val <= list2.val:
                current.next = list1
                list1 = list1.next
            else:
                current.next = list2
                list2 = list2.next
            current = current.next

        current.next = list1 or list2
        return dummy.next
```

## Complexity

| Time     | Space |
| -------- | ----- |
| O(m + n) | O(1)  |

## Tags

[Grind 75](/catalog/grind-75), [Grind](/catalog/grind), [Blind 75](/catalog/blind-75), [NeetCode 150](/catalog/neetcode-150), [NeetCode 250](/catalog/neetcode-250), [NeetCode All](/catalog/neetcode).