21.合并两个有序链表

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        if(l1==null){
            return l2;
        }
        if(l2==null)
        {
            return l1;
        }


        ListNode L1Head=l1;
        ListNode L2Head=l2;

        ListNode NewHead=new ListNode(-9);
        ListNode NewHeadPtr=NewHead;

        while(L1Head!=null && L2Head!=null)
        {
            if(L1Head.val<=L2Head.val)
            {
                NewHeadPtr.next=L1Head;

                L1Head=L1Head.next;
                NewHeadPtr=NewHeadPtr.next;
            }
            else{
                 NewHeadPtr.next=L2Head;

                L2Head=L2Head.next;
                NewHeadPtr=NewHeadPtr.next;
                }
        }

        if(L2Head!=null)
        {
            NewHeadPtr.next=L2Head;
        }
        else if(L1Head!=null)
        {
            NewHeadPtr.next=L1Head;
        }

        return NewHead.next;

        // if(L1Head.val<L2Head.val)//那一个小 就插哪一个
        // {
            
        // }else{

        // }

        
    }
}