2.Add Two Numbers

Tags: Medium Link List

Link: https://leetcode.com/problems/add-two-numbers/

---

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.

---

Answer:

/**
 * 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 head(0), *p = &head;
        int extra = 0;

        while(l1 || l2 || extra){
            int sum = (l1 ? l1 -> val : 0) + (l2 ? l2 -> val : 0) + extra;
            extra = sum / 10;
            p -> next = new ListNode(sum % 10);
            p = p -> next;
            l1 = l1 ? l1 -> next : l1;
            l2 = l2 ? l2 -> next : l2;
        }

        return head.next;
    }
};

解析：

相当于竖式加法。竖式加法总是把两个数的位数对齐，从各位开始相加，大于10进1。所以我们需要一个sum来保留当前位相加的结果，需要extra来记录是否进位。