2. Add Two Numbers

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一，题目

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

二，思路

1，计算链表l1代表的数字2，计算链表l2代表的数字3，对两个链表的值相加4，根据得到的值从个位开始，一个一个的取出创建链表注意：int类型值的溢出，这里使用long

另外结果也要是倒序的

三，代码如下

public ListNode addTwoNumbers(ListNode l1, ListNode l2) { 
        ListNode prev = new ListNode(0); 
        ListNode head = prev; 
        int carry = 0; 
        while (l1 != null || l2 != null || carry != 0) { 
            ListNode cur = new ListNode(0); 
            int sum = ((l2 == null) ? 0 : l2.val) + ((l1 == null) ? 0 : l1.val) + carry; 
            cur.val = sum % 10; 
            carry = sum / 10; 
            prev.next = cur; 
            prev = cur;        l1 = (l1 == null) ? l1 : l1.next;        l2 = (l2 == null) ? l2 : l2.next;    }    return head.next;}

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