You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: 21. Merge Two Sorted Lists.go
    path of file: ./repos/LeetCode-Go/leetcode/0021.Merge-Two-Sorted-Lists
    
    the code of the file until where you have to start completion: package leetcode

import (
	"github.com/halfrost/LeetCode-Go/structures"
)

// ListNode define
type ListNode = structu
    
func mergeTwoLists(l1 *ListNode, l2 *ListNode) *ListNode {
	if l1 == nil {
		return l2
	}
	if l2 == nil {
		return l1
	}
	if l1.Val < l2.Val {
		l1.Next = mergeTwoLists(l1.Next, l2)
		return l1
	}
	l2.Next = mergeTwoLists(l1, l2.Next)
	return l2
}