21. 合并两个有序链表

输入：1->2->4, 1->3->4
输出：1->1->2->3->4->4

/**
 * 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) {
        //首节点
        ListNode listNode = new ListNode(0);
        ListNode p = listNode;
        while (l1 != null && l2 != null) {
            if (l1.val <= l2.val) {
                p.next = l1;
                l1 = l1.next;
            } else {
                p.next = l2;
                l2 = l2.next;
            }
            p = p.next;
        }
        if (l1 != null) {
            p.next = l1;
        }
        if (l2 != null) {
            p.next = l2;
        }
        return listNode.next;
    }
}