2. Add Two Numbers

Description

You are given two non-empty linked lists representing two non-negative integers. 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.
You may assume the two numbers do not contain any leading zero, except the number 0 itself.
输入为两个非空链表代表两个非负的整数。这些整数以逆序的形式存储，并且每个元素包含一个一位数。将这两个整数相加得到一个新的链表并返回。
可以假定两个整数没有前导0，除了整数0本身。

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */

samples

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

用链表运算来表示加法，即342+465=807

Solutions

1. O(n)

   // com.cnsumi.leetcode.algorithms. A0002_Add_Two_Numbers.java;
   class ListNode {
       int val;
       ListNode next;
       public ListNode(int val) {
           this.val = val;
       }
       
       @Override
       public String toString() {
           StringBuilder ret = new StringBuilder();
           ret.append("(");
           ListNode node = this;
           do {
               if (node != this) {
                   ret.append("->");   
               }
               ret.append(node.val);
               node = node.next;
           } while (node != null);
           ret.append(")");
           return ret.toString();
       }
   }
   
   public class A0002_Add_Two_Numbers {
       public static void main(String[] args) {
           ListNode l1 = new ListNode(2);
           l1.next = new ListNode(4);
           l1.next.next = new ListNode(3);
           ListNode l2 = new ListNode(5);
           l2.next = new ListNode(6);
           l2.next.next = new ListNode(4);
           System.out.println(l1);
           System.out.println(l2);
           System.out.println(solution1(l1, l2));
       }
       
       public static ListNode solution1(ListNode l1, ListNode l2) {
           int carry = 0;
           ListNode head = new ListNode(0);
           ListNode pre = head;
           while (l1 != null || l2 != null || carry > 0) {
               int sum = (l1 == null ? 0 : l1.val) + (l2 == null ? 0 : l2.val) + carry;
               carry = sum / 10;
               pre.next = new ListNode(sum % 10);
               pre = pre.next;
               l1 = l1 == null ? l1 : l1.next;
               l2 = l2 == null ? l2 : l2.next;
           }
           return head.next;
       }
   }