package main

type ListNode struct {
	Val  int
	Next *ListNode
}

/**
 * Definition for singly-linked list.
 * type ListNode struct {
 *     Val int
 *     Next *ListNode
 * }
 */
func mergeTwoLists(list1 *ListNode, list2 *ListNode) *ListNode {
	next1, next2 := list1, list2
	head := &ListNode{
		Val:  -1,
		Next: nil,
	}
	current := head
	for next1 != nil || next2 != nil {
		if next2 == nil || (next1 != nil && next1.Val < next2.Val) {
			current.Next = next1
			next1 = next1.Next
		} else {
			current.Next = next2
			next2 = next2.Next
		}
		current = current.Next
	}
	return head.Next
}