Leetcode 2:Add Two Numbers

Problem

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

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Answers

Iterate the two lists simultaneously in one loop, and calculate the ans bit by bit.
hint:
1,Don’t forget the carry number.
2,The two lists are not the same length.

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Code

/**
 * 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 = NULL, *last = NULL;
        int carry = 0;
        for (ListNode *add1 = l1, *add2 = l2; add1 != NULL || add2 != NULL;) {
            int value = carry;
            if (add1 != NULL){
                value += add1->val;
                add1 = add1->next;
            } 
            if (add2 != NULL) {
                value += add2->val;
                add2 = add2->next;
            }
            if (value >= 10) {
                value -= 10;
                carry = 1;
            } else carry = 0;
            if (last == NULL) {
                head = last = new ListNode(value);
            } else {
                last->next = new ListNode(value);
                last = last->next;
            }

        }
        if (carry == 1) last->next = new ListNode(1);
        return head;
    }
};