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; } }