21. Merge Two Sorted Lists

type ListNode struct {
     Val int
     Next *ListNode
 }
func mergeTwoLists(l1 *ListNode, l2 *ListNode) *ListNode {
    if l1 == nil {
        return l2
    }
    if l2 == nil {
        return l1
    }
    head := &ListNode{}
    p := head
    for l1 != nil && l2 != nil {
        if l1.Val < l2.Val {
            p.Next = l1
            l1 = l1.Next
        } else {
            p.Next = l2
            l2 = l2.Next
        }
        p = p.Next
    }
    if l1 != nil {
        p.Next = l1
    }
    if l2 != nil {
        p.Next = l2
    }
    return head.Next
}