【LeetCode】2. Add Two Numbers

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博客围绕两非空链表表示非负整数相加展开，数字以逆序存储在链表节点，每个节点含一位数，需将两数相加并以链表形式返回，还提醒熟悉链表使用及头节点好处。

2. Add Two Numbers

Medium

You are given two non-empty linked lists representing two non-negative integers. 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.

You may assume the two numbers do not contain any leading zero, except the number 0 itself.

Example:

Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8
Explanation: 342 + 465 = 807.

熟悉链表使用，注意头节点（虚结点）的好处。

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
        ListNode ln(-1);    //这个结点是虚结点
        ListNode* ptr = &ln;    
        int sum = 0;int carry = 0;
        while(l1||l2||carry)
        {
            if(l1){
                sum+=l1->val;
                l1=l1->next;
            }
            if(l2){
                sum+=l2->val;
                l2=l2->next;
            }
            if(carry){
                sum+=carry;
                carry=0;
            }
            carry=sum/10;
            ptr->next = new ListNode(sum%10);
            ptr = ptr->next;
            sum = 0;
        }
        ptr->next = NULL;//最后一个结点置为NULL
        ptr = ln.next;//最后通过虚结点返回结果
        return ptr;
    }
};