Add Two Numbers as Lists

• Time : $O(max(|A|,|B|))$
• Space : $O(max(|A|,|B|))$

c++

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
ListNode* Solution::addTwoNumbers(ListNode* A, ListNode* B) {
    ListNode* dummy = new ListNode(-1);
    ListNode* runner = dummy;
    int carry = 0;
    while(A and B) {
        int sum = A->val + B->val + carry;
        carry = sum / 10;
        runner->next = new ListNode(sum%10);
        runner = runner->next;
        A = A->next;
        B = B->next;
    }
    
    while(A) {
        int sum = A->val + carry;
        carry = sum / 10;
        runner->next = new ListNode(sum%10);
        runner = runner->next;
        A = A->next;
    }

    while(B) {
        int sum = B->val + carry;
        carry = sum / 10;
        runner->next = new ListNode(sum%10);
        runner = runner->next;
        B = B->next;
    }
    
    if(carry) runner->next = new ListNode(carry);
    
    return dummy->next;
}