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

1 刷
Time Complexity - O(n)，Space Complexity - O(1)。

public class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        if(l1 == null || l2 == null) return null;
        ListNode res = new ListNode(-1);
        ListNode node = res;
        int carry = 0;
        while(l1!=null || l2!=null){
            int digitL1 = 0, digitL2 = 0;
            if(l1!=null){
                digitL1 = l1.val;
                l1 = l1.next;
            }
            if(l2!=null){
                digitL2 = l2.val;
                l2 = l2.next;
            }
            
            int newNum = digitL1 + digitL2 + carry;
            node.next = new ListNode(newNum%10);
            carry = newNum >= 10? 1 : 0;
            node = node.next;
        }
        if(carry == 1) node.next = new ListNode(1);
        return res.next;
    }
}