LeetCode: Add Two Numbers 解题报告

Add Two Numbers
You are given two linked lists representing two non-negative numbers. The digits are stored in reverse order and each of their nodes contain a single digit. Add the two numbers and return it as a linked list.

Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8

https://oj.leetcode.com/problems/add-two-numbers/

解答：

比较简单，直接用2个指针往后移动，并且将2个list的值相加，并且计算上carry值。注意，在while循环中加上carry == 1的判断，这样可以自动在链尾加上carry 的值。

 1 package Algorithms.list;
 2
 3 import Algorithms.algorithm.others.ListNode;
 4
 5 /**
 6  * Definition for singly-linked list.
 7  * public class ListNode {
 8  *     int val;
 9  *     ListNode next;
10  *     ListNode(int x) {
11  *         val = x;
12  *         next = null;
13  *     }
14  * }
15  */
16 public class AddTwoNumbers {
17     public static void main(String[] str) {
18         ListNode n1 = new ListNode(9);
19
20         ListNode l1 = new ListNode(1);
21         ListNode l2 = new ListNode(9);
22         ListNode l3 = new ListNode(9);
23         ListNode l4 = new ListNode(9);
24         ListNode l5 = new ListNode(9);
25         ListNode l6 = new ListNode(9);
26         ListNode l7 = new ListNode(9);
27         ListNode l8 = new ListNode(9);
28         ListNode l9 = new ListNode(9);
29         ListNode l10 = new ListNode(9);
30
31         l1.next = l2;
32         l2.next = l3;
33         l3.next = l4;
34         l4.next = l5;
35         l5.next = l6;
36         l6.next = l7;
37         l7.next = l8;
38         l8.next = l9;
39         l9.next = l10;
40
41         System.out.println(addTwoNumbers(n1, l1).toString());
42     }
43
44     public static ListNode addTwoNumbers(ListNode l1, ListNode l2) {
45         if (l1 == null || l2 == null) {
46             return null;
47         }
48
49         ListNode dummy = new ListNode(0);
50         ListNode tail = dummy;
51         int carry = 0;
52
53         while (l1 != null || l2 != null || carry == 1) {
54             int sum = carry;
55             if (l1 != null) {
56                 sum += l1.val;
57                 l1 = l1.next;
58             }
59
60             if (l2 != null) {
61                 sum += l2.val;
62                 l2 = l2.next;
63             }
64
65             carry = sum / 10;
66
67             // create a new node and add it to the tail.
68             ListNode cur = new ListNode(sum % 10);
69             tail.next = cur;
70             tail = tail.next;
71         }
72
73         return dummy.next;
74     }
75 }

代码：

AddTwoNumbers