[LeetCode] Add Two Numbers

The idea is to add the two numbers (represented by linked lists) nodes after nodes and store the result in the longer list. Since we may append the additioanl carry bit after the longer list, we need to be careful not to reach the NULL pointer of the longer list.

The code is as follows. In fact, it is of time O(length(l1) + length(l2)) (the two calls oflistLength) and includes two-pass, but it is very fast in the OJ :-)

The code is as follows, about 20 lines. You may run it on the following examples and then you will understand the details of the code.

class Solution {
public:
    ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
        if (listLength(l1) < listLength(l2))
            return addTwoNumbers(l2, l1);
        ListNode *r1 = l1, *r2 = l2;
        int c = 0;
        bool isEnd = false;
        while (r2) {
            int val = r1 -> val + r2 -> val + c;
            r1 -> val = val % 10;
            c = val / 10;
            if (r1 -> next) r1 = r1 -> next;
            else isEnd = true;
            r2 = r2 -> next;
        }
        while (c) {
            int val = isEnd ? c : r1 -> val + c;
            if (isEnd) r1 -> next = new ListNode(val % 10);
            else r1 -> val = val % 10;
            c = val / 10;
            if (r1 -> next) r1 = r1 -> next;
            else isEnd = true;
        }
        return l1;
    }
private:
    int listLength(ListNode* head) {
        return head ? 1 + listLength(head -> next) : 0;
    }
};

Examples:

1. l1 = 5 -> NULL, l2 = 5 -> NULL;
2. l1 = 1 -> NULL, l2 = 9 -> 9 -> NULL;
3. l1 = 8 -> 9 -> NULL, l2 = 1 - >NULL.

Well, you guess what? Yeah, these are just the test cases which my code gives mistake before the debugging :-)