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.

Return the head of the merged linked list.

Function signature

func MergeTwoLists(list1 *ListNode, list2 *ListNode) *ListNode

ListNode definition

type ListNode struct {
    Val  int
    Next *ListNode
}

Examples

Example 1:

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

list1:  1 -> 2 -> 4
list2:  1 -> 3 -> 4
merged: 1 -> 1 -> 2 -> 3 -> 4 -> 4

Example 2:

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

Example 3:

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

Hints

1. Use a dummy head node to simplify edge cases.
2. Compare current nodes from both lists, attach the smaller one.
3. When one list is exhausted, attach the rest of the other list.

Notes

• This is an in-place merge using O(1) extra space.
• The dummy head technique avoids special-casing the head of the result.
• This is a building block for merge sort on linked lists.