leetcode 21. Merge Two Sorted Lists

题目

Merge two sorted linked lists and return it as a new list. The new list should be made by splicing together the nodes of the first two lists.

解1

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


        if(l1==null && l2==null){
            return null;
        }
        if(l1==null && l2!=null){
            return l2;
        }
        if(l1!=null && l2==null){
            return l1;
        }

        ListNode p=l1;
        ListNode q=l2;
        ListNode head=null;
        ListNode end=null;
        ListNode base=null;//指向head的指针
        if(p.val<=q.val){
            head=l1;
            p=p.next;
        }else{
            head=l2;
            q=q.next;
        }

        base=head;

        while(p!=null&&q!=null){
            if(p.val<=q.val){
                base.next=p;
                p=p.next;
            }else{
                base.next=q;
                q=q.next;
            }           
            base=base.next;
        }
        if(p!=null){
            base.next=p;
        }
        if(q!=null){
            base.next=q;
        }

        return head;

    }
}

解2（优解）

   public ListNode mergeTwoLists(ListNode l1, ListNode l2) {//递归的方式
        if(l1==null){
            return l2;
        }else if(l2==null)
            return l1;

        ListNode pMergeHead=null;

        if(l1.val<l2.val){
            pMergeHead=l1;
            pMergeHead.next=mergeTwoLists(l1.next,l2);
        }else{
            pMergeHead=l2;
            pMergeHead.next=mergeTwoLists(l1,l2.next);
        }

        return pMergeHead;

    }