165 - 合并两个排序链表

5.4

很好玩儿的是，我如果这样写就是错误的。

else if(l1.val == l2.val){
flag.next = l1;
flag = flag.next;
flag.next = l2;
flag = flag.next;
l1 = l1.next;
l2 = l2.next;
}

这个错误找了半天才找到。这是由于flag.next的操作会改变l2链表。
应该在flag操作之前，对原有链表进行操作。

/**
* 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) {
// write your code here

if(l1 == null){
return l2;
}
if(l2 == null){
return l1;
}
//把l1 当头结点，在l1 的基础上操作
ListNode head = new ListNode(0);
head.next = null;
ListNode flag = head;
while(l1 != null && l2 != null){
//System.out.println("l1:" + l1.val + ";l2:" + l2.val);
if(l1.val < l2.val){
flag.next = l1;
l1 = l1.next;
flag = flag.next;
}
else if(l1.val == l2.val){
flag.next = l1;
l1 = l1.next;
flag = flag.next;
flag.next = l2;
l2 = l2.next;
flag = flag.next;
}
else {
flag.next = l2;
flag = flag.next;
l2 = l2.next;
}
}
while(l1 != null){
//System.out.println("l1:" + l1.val);
flag.next = l1;
flag = flag.next;
l1 = l1.next;
// System.out.println(l1.val);

}
while(l2 != null){
// System.out.println("l2:" + l2.val);
flag.next = l2;
l2 = l2.next;
flag = flag.next;
}
return head.next;
}
}