合并两个排序链表

/**
 * Definition for ListNode.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int val) {
 *         this.val = val;
 *         this.next = null;
 *     }
 * }
 */ 
public class Solution {
    /**
     * @param ListNode l1 is the head of the linked list
     * @param ListNode l2 is the head of the linked list
     * @return: ListNode head of linked list
     */
	public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
		ListNode listNode = new ListNode(0);
		ListNode lastNode = listNode;
		while (l1 != null && l2 != null) {
			if (l1.val > l2.val) {
				lastNode.next = l2;
				l2 = l2.next;
			} else {
				lastNode.next = l1;
				l1 = l1.next;
			}
			lastNode = lastNode.next;
		}

		if (l1 == null)
			lastNode.next = l2;

		if (l2 == null) {
			lastNode.next = l1;
		}

		return listNode.next;
	}
}