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.

Example:

Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8
Explanation: 342 + 465 = 807.

为了方便以后测试ListNode添加了额外的代码。

public class ListNode {
    int val;
    ListNode next;
    
    ListNode(int x) {
        val = x;
    }
    
    @Override
    public String toString() {
        StringBuilder s = new StringBuilder(String.valueOf(val));
        ListNode temp = next;
        while(temp != null){
            s.append(",").append(temp.val);
            temp = temp.next;
        }
        return s.toString();
    }

    public void addLast(int val){
        ListNode temp = this;
        while(temp.next != null){
            temp = temp.next;
        }
        temp.next = new ListNode(val);
    }
}

简单的小学加法逻辑：

class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        ListNode head = new ListNode(0);
        ListNode current = head;
        int flag = 0;
        while (flag != 0 || (l1 != null && l2 != null)){
            int val = (l1 == null ? 0 : l1.val) + (l2 == null ? 0 : l2.val) + flag;
            flag = val > 9 ? 1 : 0;
            current.next = new ListNode(val%10);
            current = current.next;
            l1 = (l1 == null) ? null : l1.next;
            l2 = (l2 == null) ? null : l2.next;
        }
        current.next = (l1 != null) ? l1 : l2;
        return head.next;
    }
}