21. 合并两个有序链表

    /**
     * Definition for singly-linked list.
     * type ListNode struct {
     *     Val int
     *     Next *ListNode
     * }
     */
    func mergeTwoLists(l1 *ListNode, l2 *ListNode) *ListNode {
        if l1 == nil {
            return l2
        }
        if l2 == nil {
            return l1
        }
        var res *ListNode
        if l1.Val < l2.Val {
            res = &ListNode{
                Val: l1.Val,
            }
            l1 = l1.Next
        } else {
            res = &ListNode{
                Val: l2.Val,
            }
            l2 = l2.Next
        }
        head := res
        for l1 != nil && l2 != nil {
            if l1.Val < l2.Val {
                res.Next = &ListNode{
                    Val: l1.Val,
                }
                l1 = l1.Next
            } else {
                res.Next = &ListNode{
                    Val: l2.Val,
                }
                l2 = l2.Next
            }
            res = res.Next
        }
        if l1 == nil {
            res.Next = l2
        } else {
            res.Next = l1
        }
        return head
    }