Add Two Numbers @LeetCode

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

	package leetcode.linkedlist;
	/**
	* Solution: Just add from head to rear and consider carry. Process is same to merge two sorted LinkedList
	* head is single digit. then ten digit, hundred digit.
	* I was confused by "The digits are stored in reverse order" at first. Don't need to reverse any more.
	*
	* @author jeffwan
	* @date Apr 1, 2014
	*/
	public class AddTwoNumbers {
	public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
	if (l1 == null && l2 == null) {
	return null;
	}
	ListNode dummy = new ListNode(0);
	ListNode head = dummy;
	int carry = 0;
	int sum = 0;
	while (l1 != null && l2 != null) {
	sum = l1.val + l2.val + carry;
	carry = sum / 10;
	head.next = new ListNode(sum % 10);
	l1 = l1.next;
	l2 = l2.next;
	head = head.next;
	}
	while (l1 != null) {
	sum = l1.val + carry;
	carry = sum / 10;
	head.next = new ListNode(sum % 10);
	l1 = l1.next;
	head = head.next;
	}
	while (l2 != null) {
	sum = l2.val + carry;
	carry = sum / 10;
	head.next = new ListNode(sum % 10);
	l2 = l2.next;
	head = head.next;
	}
	if (carry != 0) {
	head.next = new ListNode(carry);
	}
	return dummy.next;
	}
	// ListNode class
	public static class ListNode {
	int val;
	ListNode next;
	ListNode(int x) {
	val = x;
	next = null;
	}
	}
	}

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