Question:

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

解决：

题目是指将两数相加，但是数字表示逆序。相加时需要注意和大于10时需要进位，这也是代码最后部分进行判断的原因。

public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
    ListNode sum = new ListNode(0);
    ListNode p = l1;
    ListNode q = l2;
    ListNode r = sum;
    int carry = 0;
    while(p != null || q != null){
        int d1 = p != null ? p.val : 0;
        int d2 = q != null ? q.val : 0;
        d1 += carry + d2;
        carry = d1 / 10;
        r.next = new ListNode(d1 % 10);
        r = r.next;
        if (p != null)
            p = p.next;
        if (q != null)
            q = q.next;
    }
    if (carry == 1)
        r.next = new ListNode(1);
    return sum.next;
}

代码地址（附测试代码）：
https://github.com/shichaohao/LeetCodeUsingJava/tree/master/src/addTwoNumbers