leetcode 5. 两个链表逐个元素相加 Add Two Numbers

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7问题：Add Two Numbers 难度-Medium

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

解答：
目标：做一个带进位的加法器（243+564 = 708）
本题需要重点考虑的是–两个链表不等长的情况和最后的进位

class Solution {public:    ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {        if (l1 == NULL || l2 == NULL)           return l1 == NULL ? l2 : l1;            ListNode dummy(-1); //虚拟节点，指向链表头        ListNode* l = &dummy;        int carry = 0;        int v1, v2, sum;        while (l1 || l2) {            if (l1) {                 v1 = l1->val;                l1 = l1->next;            } else {                v1 = 0;            }            if (l2) {                v2 = l2->val;                l2 = l2->next;            } else {                v2 = 0;            }            sum = v1 + v2 + carry;            l->next = new ListNode(sum % 10);//后插法，如果是C语言，写一个后插函数            carry = sum / 10;            l = l->next;        }        //l1,l2均达到尾部时，处理最后的进位        if (carry) {            l->next = new ListNode(carry);        }        return dummy.next;              }};

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