2. Add Two Numbers

该题直接模拟加法过程。每个node的值是一个单一的数字，且没有前置0，条件设置相对简单，直接扫描两个链表即可。

/** * Definition for singly-linked list. * public class ListNode { *     int val; *     ListNode next; *     ListNode(int x) { val = x; } * } */class Solution {    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {        if (l1 == null||l2 == null){            return null;        }        int carry = 0, sum = 0;        ListNode retNode = new ListNode(0);        ListNode node = retNode;        while (l1 != null&&l2 != null){            sum = l1.val + l2.val + carry;            ListNode tmpNode = new ListNode(sum%10);            carry = sum/10;            node.next = tmpNode;            node = node.next;            l1 = l1.next;            l2 = l2.next;        }        while (l1 != null){            sum = l1.val + carry;            ListNode tmpNode = new ListNode(sum%10);            carry = sum/10;            node.next = tmpNode;            node = node.next;            l1 = l1.next;        }        while (l2 != null){            sum = l2.val + carry;            ListNode tmpNode = new ListNode(sum%10);            carry = sum/10;            node.next = tmpNode;            node = node.next;            l2 = l2.next;        }        if (carry != 0){            ListNode tmpNode = new ListNode(carry);            node.next = tmpNode;        }        return retNode.next;    }}