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。

 package Algorithms.list;

 import Algorithms.algorithm.others.ListNode;

 /**
  * Definition for singly-linked list.
  * public class ListNode {
  *     int val;
  *     ListNode next;
  *     ListNode(int x) {
  *         val = x;
  *         next = null;
  *     }
  * }
  */
 public class AddTwoNumbers {
     public static void main(String[] str) {
         ListNode n1 = new ListNode(9);

         ListNode l1 = new ListNode(1);
         ListNode l2 = new ListNode(9);
         ListNode l3 = new ListNode(9);
         ListNode l4 = new ListNode(9);
         ListNode l5 = new ListNode(9);
         ListNode l6 = new ListNode(9);
         ListNode l7 = new ListNode(9);
         ListNode l8 = new ListNode(9);
         ListNode l9 = new ListNode(9);
         ListNode l10 = new ListNode(9);

         l1.next = l2;
         l2.next = l3;
         l3.next = l4;
         l4.next = l5;
         l5.next = l6;
         l6.next = l7;
         l7.next = l8;
         l8.next = l9;
         l9.next = l10;

         System.out.println(addTwoNumbers(n1, l1).toString());
     }

     public static ListNode addTwoNumbers(ListNode l1, ListNode l2) {
         if (l1 == null || l2 == null) {
             return null;
         }

         ListNode dummy = new ListNode(0);
         ListNode tail = dummy;
         int carry = 0;

         while (l1 != null || l2 != null || carry == 1) {
             int sum = carry;
             if (l1 != null) {
                 sum += l1.val;
                 l1 = l1.next;
             }

             if (l2 != null) {
                 sum += l2.val;
                 l2 = l2.next;
             }

             carry = sum / 10;

             // create a new node and add it to the tail.
             ListNode cur = new ListNode(sum % 10);
             tail.next = cur;
             tail = tail.next;
         }

         return dummy.next;
     }
 }

代码：

AddTwoNumbers