/*
 * @lc app=leetcode id=445 lang=java
 *
 * [445] Add Two Numbers II
 */

// @lc code=start
/**
 * 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) {
        l1 = reverse(l1);
        l2 = reverse(l2);
        return reverse(addByLittleEnd(l1, l2));
    }

    private ListNode addByLittleEnd(ListNode l1, ListNode l2) {
        ListNode dummy = new ListNode(0), tail = dummy;
        dummy.next = null;
        boolean flow = false;
        while(l1 != null || l2 != null) {
            int v1 = l1==null?0:l1.val;
            int v2 = l2==null?0:l2.val;
            int v = v1 + v2 + (flow?1:0);
            flow = v>=10;
            ListNode tmp = new ListNode(v%10, null);
            tail.next = tmp;
            tail = tail.next;
            tail.next = null;
            if(l1 != null) {
                l1 = l1.next;
            } 
            if(l2 != null) {
                l2 = l2.next;
            }
        }
        if(flow) {
            ListNode tmp = new ListNode(1, null);
            tail.next = tmp;
            tail = tail.next;
            tail.next = null;
        }
        return dummy.next;
    }

    private ListNode reverse(ListNode l) {
        if(l == null) {
            return l;
        }
        ListNode dummy = new ListNode(0), tmp;
        dummy.next = null;
        while(l != null) {
            tmp = l.next;
            l.next = dummy.next;
            dummy.next = l;
            l = tmp;   
        }
        return dummy.next;
    }
}
// @lc code=end