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
给定两组链表，分别代表两组非负数。数字以反序存储，并且每一个点只包含一个数字。将这两组数相加，以链表的形式返回。

答案

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        ListNode p = l1;
        ListNode q = l2;
        ListNode resultNode = new ListNode(0);//创建链表相加后第一个链表
        ListNode cur = resultNode;//此时cur也指向第一个链表
        
        int carry = 0;
        while(p!=null || q!=null) {
            int x = (p != null)? p.val:0;
            int y = (q != null)? q.val:0;
            int sum = x + y +carry;
            carry = sum / 10;
            cur.next = new ListNode(sum % 10);
            cur = cur.next;
            
            if (p != null) p = p.next;
            if (q != null) q = q.next;
        }
        
        if (carry > 0) cur.next = new ListNode(carry);//如果最后一位大于10，新增加一个链表
        
        return resultNode.next;//此时cur已经指向了整个结果链表的最后一位，因此返回应该返回还指向整个结果链表首位的resultNode.next
                               //如果是return resultNode,则最后结果的首位多一个0，变成了四位
    }
}

参考

Add Two Numbers