21. 合并两个有序链表

题目：
将两个升序链表合并为一个新的 升序 链表并返回。新链表是通过拼接给定的两个链表的所有节点组成的。

解法：

/**
 * Definition for singly-linked list.
 * type ListNode struct {
 *     Val int
 *     Next *ListNode
 * }
 */
func mergeTwoLists(list1 *ListNode, list2 *ListNode) *ListNode {
    head1, head2 := list1, list2
    var dummy = &ListNode{}
    head := dummy
    for head1 != nil && head2 != nil {
        if head1.Val < head2.Val {
            head.Next = head1
            head1 = head1.Next
        } else {
            head.Next = head2
            head2 = head2.Next
        }
        head = head.Next
    }

    if head1 != nil {
            head.Next = head1
    } else {
            head.Next = head2
    }
    

    return dummy.Next
}