LeetCode之Merge Two Sorted Lists

/**
 * 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) {
        if (l1 == null) {
      return l2;
    }
    if (l2 == null) {
      return l1;
    }
    ListNode head = new ListNode(0);
    ListNode cur = head;
    while (l1 != null && l2 != null) {
      if (l1.val <= l2.val) {
        //cur.val = l1.val;这样写会爆空指针异常
        cur.next = l1;
        l1 = l1.next;
      } else {
        //cur.val = l2.val;这样写会爆空指针异常
        cur.next = l2;
//        System.out.println(head.val);
        l2 = l2.next;
      }
      cur = cur.next;
    }
    if (l1 != null) {
      cur.next = l1;
    } else {
      System.out.println("l2 != null");
      cur.next = l2;
    }
        return head.next;
    }
}