Leetcode

Add Two Numbers

 
  • Feb 01, 2020

Problem

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.

Leetcode: English, Chinese

Solution

Just simulate. Use a dummy node to avoid extra conditional statements for initializing the head’s value.

Code

Github (C++)

/**
 * 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* dummy = new ListNode(0);
        ListNode* p = dummy;
        int carry = 0;

        while (l1 || l2 || carry) {
            int x = l1 ? l1->val : 0;
            int y = l2 ? l2->val : 0;

            int sum = x + y + carry;
            carry = sum / 10;

            p->next = new ListNode(sum % 10);
            p = p->next;

            if (l1) l1 = l1->next;
            if (l2) l2 = l2->next;
        }
        return dummy->next;
    }
};
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