Add Two Numbers

Add the two numbers and return it as a linked list.

Question

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.

Analyzation

1. Since the numbers are added from the first to the last, which makes the problem simpler. We only need to create a new LinkedList head(dummy) that points to null and a tail that points to the first node of the first added number which is (2 + 5) = 7.
2. The key part is make sure if there is a leading “1” after the sum. If the sum has a leading “1”, then the LinkedList should not end.

The Code

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        // create a new linked list that adds two digits and moves on
        int sum = 0;
        ListNode dummy = new ListNode(-1);
        ListNode tail = head;
        
        // loop till both listNodes are pointing to null or sum is 0
        while(l1 != null || l2 != null || sum != 0){
            if(l1 != null){
                sum += l1.val;
                l1 = l1.next;
            }
            if(l2 != null){
                sum += l2.val;
                l2 = l2.next;
            }
            tail.next = new ListNode(sum % 10);  // get the one's position
            sum /= 10;  // check to see if there is a leading 1
            tail = tail.next;  // moves to the next node
        }
        return dummy.next;
    }
}

Time & Space Complexity

• Time Complexity: Since we create a loop so it is O(n).
• Space Complexity: Since we use an extra memory which is LinkedList, the Space Complexity is also O(n).