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.

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

Solution

Two-pointers

别忘记最高一位产生的carry！

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        ListNode dummy = new ListNode(0);
        int carry = 0;
        ListNode tail = dummy;
        
        while (l1 != null || l2 != null) {
            int val = carry;
            if (l1 != null) {
                val += l1.val;
                l1 = l1.next;
            }
            
            if (l2 != null) {
                val += l2.val;
                l2 = l2.next;
            }
            
            tail.next = new ListNode(val % 10);
            carry = val / 10;
            tail = tail.next;
        }
       
        if (carry > 0) {   // important!
            tail.next = new ListNode(carry);
        }
        
        return dummy.next;
    }
}