Leetcode - 002 - Add Two Numbers

2. Add Two Numbers

• Medium

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 contains a single digit. Add the two numbers and return the sum as a linked list.

You may assume the two numbers do not contain any leading zero, except the number 0 itself.

Example 1:
Input: l1 = [2,4,3], l2 = [5,6,4]
Output: [7,0,8]
Explanation: 342 + 465 = 807.

Example 2:
Input: l1 = [0], l2 = [0]
Output: [0]

Example 3:
Input: l1 = [9,9,9,9,9,9,9], l2 = [9,9,9,9]
Output: [8,9,9,9,0,0,0,1]

Constraints:
    The number of nodes in each linked list is in the range [1, 100].
    0 <= Node.val <= 9
    It is guaranteed that the list represents a number that does not have leading zeros.

Solution

1. Create a ListNode to store the additive results and an integer variable i to store the carry
2. Traverse the ListNodes, additive for each node
3. If the result of the node > 10, a number greater than a single digit is put into i
4. When the ListNode to be added is empty (the accumulation is complete) and the i = 0 (no carry), return the result ListNode

Java

class ListNode {
    public int val;
    public ListNode next;
    public ListNode() {}
    public ListNode(int val) { this.val = val; }
    public ListNode(int val, ListNode next) { this.val = val; this.next = next; }
}

public class lc002 {
    public static ListNode addTwoNumbers(ListNode ln1, ListNode ln2) {
        ListNode ln0 = new ListNode(0);
        ListNode lnres = ln0;
        ListNode lntmp;

        int i = 0;
        while (true) {
            lntmp = ln1;
            if (lntmp !=null) {
                ln0.val += lntmp.val;
                lntmp = lntmp.next;
            }
            ln1 = lntmp;

            lntmp = ln2;
            if (lntmp !=null) {
                ln0.val += lntmp.val;
                lntmp = lntmp.next;
            }
            ln2 = lntmp;

            while (ln0.val > 9) {
                ln0.val -= 10;
                i++;
            }

            if (ln1 != null || ln2 != null || i>0) {
                ln0.next = new ListNode(0);
                ln0 = ln0.next;
                ln0.val = i;
                i = 0;
            } else {
                break;
            }
        }

        return lnres;
    }


    public static ListNode ListNodeInit(ArrayList<Integer> l1) {
        ListNode lnres, lntmp;
        lntmp = new ListNode(l1.get(0));
        lnres = lntmp;
        for (int i=1; i<l1.size(); i++) {
            lntmp.next = new ListNode(l1.get(i));
            lntmp = lntmp.next;
        }
        return lnres;
    }

    public static void main(String[] args) {
        ListNode ln1, ln2;

        ArrayList<Integer> l1, l2;
        l1 = new ArrayList<>(Arrays.asList(2,3,4));
        l2 = new ArrayList<>(Arrays.asList(5,6,4));

        System.out.println(l1);
        System.out.println(l2);

        ln1 = ListNodeInit(l1);
        ln2 = ListNodeInit(l2);

        ListNode lnres = addTwoNumbers(ln1, ln2);

        System.out.println("lnres Node ===");
        int i = 0;
        while(lnres != null) {
            System.out.println(String.format("Node %s: %s", i++, lnres.val));
            lnres = lnres.next;
        }
    }
}