AddTwoNumbersII445.java

/**
 * You are given two non-empty linked lists representing two non-negative
 * integers. The most significant digit comes first 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.
 * 
 * Follow up:
 * What if you cannot modify the input lists? In other words, reversing the
 * lists is not allowed.
 * 
 * Example:
 * 
 * Input: (7 -> 2 -> 4 -> 3) + (5 -> 6 -> 4)
 * Output: 7 -> 8 -> 0 -> 7
 */

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */

public class AddTwoNumbersII445 {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        Stack<Integer> s1 = new Stack<>();
        Stack<Integer> s2 = new Stack<>();
        
        while (l1 != null && l2 != null) {
            s1.push(l1.val);
            l1 = l1.next;
            s2.push(l2.val);
            l2 = l2.next;
        }
        while (l1 != null) {
            s1.push(l1.val);
            l1 = l1.next;
        }
        while (l2 != null) {
            s2.push(l2.val);
            l2 = l2.next;
        }

        ListNode dummy = new ListNode(0);
        ListNode p = dummy;
        int carry = 0;
        while (!s1.isEmpty() && !s2.isEmpty()) {
            int sum = s1.pop() + s2.pop() + carry;
            ListNode curr = new ListNode(sum % 10);
            curr.next = p.next;
            p.next = curr;
            carry = sum / 10;
        }
        while (!s1.isEmpty()) {
            int sum = s1.pop() + carry;
            ListNode curr = new ListNode(sum % 10);
            curr.next = p.next;
            p.next = curr;
            carry = sum / 10;
        }
        while (!s2.isEmpty()) {
            int sum = s2.pop() + carry;
            ListNode curr = new ListNode(sum % 10);
            curr.next = p.next;
            p.next = curr;
            carry = sum / 10;
        }
        if (carry != 0) {
            ListNode curr = new ListNode(carry);
            curr.next = p.next;
            p.next = curr;
        }

        return dummy.next;
    }


    public ListNode addTwoNumbers2(ListNode l1, ListNode l2) {
      ListNode ll1 = reverseList(l1);
      ListNode ll2 = reverseList(l2);

      return reverseList(addTwoNumbers0(ll1, ll2));
    } 

    public ListNode addTwoNumbers0(ListNode l1, ListNode l2) {
        ListNode dummy = new ListNode(0);
        ListNode p = dummy;
        int carry = 0;
        while (l1 != null || l2 != null) {
            int a = l1 == null ? 0 : l1.val;
            int b = l2 == null ? 0 : l2.val;
            int sum = a + b + carry;
            ListNode n = new ListNode(sum%10);
            p.next = n;
            p = p.next;
            carry = sum/10;
            l1 = l1 == null ? null : l1.next;
            l2 = l2 == null ? null : l2.next;
        }

        if (carry != 0) p.next = new ListNode(carry);

        return dummy.next;
    }

    public ListNode reverseList(ListNode head) {
        if (head == null || head.next == null) return head;

        ListNode dummy = new ListNode(0);
        ListNode tail = null;

        while (head != null) {
            ListNode t = head;
            head = head.next;
            tail = dummy.next;
            dummy.next = t;
            dummy.next.next = tail;
        }

        return dummy.next;
    }

}