AddTwoNumbers.java

/**
 * You are given two linked lists representing two non-negative numbers.
 *
 * <p>The digits are stored in reverse order and each of their nodes contain a single digit.
 *
 * <p>Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
 *
 * <p>Output: 7 -> 0 -> 8
 */
public class AddTwoNumbers {
  public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
    if (l1 == null) return l2;
    if (l2 == null) return l1;
    ListNode head = new ListNode(0);
    ListNode cur = head;
    int plus = 0;
    while (l1 != null && l2 != null) {
      int sum = l1.val + l2.val + plus;
      plus = sum / 10;
      sum = sum % 10;
      cur.next = new ListNode(sum);
      cur = cur.next;
      l1 = l1.next;
      l2 = l2.next;
    }
    if (l1 != null) {
      if (plus != 0) {
        cur.next = addTwoNumbers(l1, new ListNode(plus));
      } else {
        cur.next = l1;
      }
    } else if (l2 != null) {
      if (plus != 0) {
        cur.next = addTwoNumbers(l2, new ListNode(plus));
      } else {
        cur.next = l2;
      }
    } else if (plus != 0) {
      cur.next = new ListNode(plus);
    }

    return head.next;
  }
}