Merge Two Sorted Lists（合并有序链表）leetcode21

code1:(常规解法)

public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        if(l1==null)
            return l2;
        if(l2==null)
            return l1;
        ListNode head=null;
        ListNode node=null;
        while(l1!=null&&l2!=null){
            if(l1.val>l2.val){
                if(head==null){
                    head=l2;
                    node=l2;
                    l2=l2.next;
                }
                else{
                node.next=l2;
                l2=l2.next;
                node=node.next;
                }
            }else{
                 if(head==null){
                    head=l1;
                     node=l1;
                     l1=l1.next;
                 }
                else{
                node.next=l1;
                l1=l1.next;
                node=node.next;
                }
            }
            
        }
        if(l1==null)
            node.next=l2;
        else
            node.next=l1;
        return head;
    }public ListNode mergeTwoLists