Merge Two Sorted Lists (Java)

題目：

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.

Example:

Input: 1->2->4, 1->3->4
Output: 1->1->2->3->4->4

題目爲：將兩個有序鏈表排序

思路：

        先設置一個帶頭結點的鏈表L3.頭節點的val值隨意。依次比較L1和l2當前結點val值的大小。若L1的val值較小，就將L1的值賦給L3。L1、L3指針向後移動一位。若L2較小，同理。直到L1或者L2某一個鏈表爲空。若L1爲空，就將L3下一節點指向L1。L2爲空，同理。返回時，返回L3頭節點之後的節點。

代碼：

/**
 * 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) {
		ListNode l3 = new ListNode(0);
		ListNode l4=l3;
		//ListNode pListNode = l1;
		//ListNode qListNode = l2;
		if(l1==null){
			return l2;
		}
		if(l2==null){
			return l1;
		}
		while(l1!=null&&l2!=null){
			if(l1.val<=l2.val){
				l3.next = l1;
				l3 = l3.next;
				l1 = l1.next;
			}else {
				l3.next = l2;
				l3 = l3.next;
				l2 = l2.next;
			}
		}
		if(l1==null){
			l3.next = l2;
		}
		if(l2==null){
			l3.next = l1;
		}
		
		return l4.next;
        
    }
}