21. 合并两个有序链表

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        ListNode returnNode = new ListNode(0);
        Merge(l1, l2, returnNode);
        return returnNode.next;
    }
    public void Merge(ListNode l1, ListNode l2, ListNode returnNode)
    {
        if(l1 == null)
        {
            returnNode.next = l2;
            return;
        }
        else if(l2 == null)
        {
            returnNode.next = l1;
            return;
        }
        else
        {
            if(l1.val <= l2.val)
            {
                returnNode.next = l1;
                ListNode tmpNode = l1.next;
                ListNode tmpNode1 = returnNode.next;
                 Merge(tmpNode, l2, tmpNode1);
            }
            else
            {
                returnNode.next = l2;
                ListNode tmpNode = l2.next;
                ListNode tmpNode1 = returnNode.next;
                Merge(l1, tmpNode, tmpNode1);
            }
        }
    }
}