2-add-two-numbers.java

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        ListNode dummy = new ListNode();
        ListNode cur = dummy;
        int carry = 0;
        while (carry > 0 || l1 != null || l2 != null) {
            if (l1 != null) {
                carry += l1.val;
                l1 = l1.next;
            }
            if (l2 != null) {
                carry += l2.val;
                l2 = l2.next;
            }
            int val = carry % 10;
            cur.next = new ListNode(val);
            cur = cur.next;
            carry /= 10;
        }
        return dummy.next;
    }
}

// time O(n)
// space O(n), creating a new linked list
// using linked list and sentinel node and two pointers