445. Add Two Numbers II

---

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

 思路：用2个stack

class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        Stack<Integer> s1 = new Stack<Integer>();
        Stack<Integer> s2 = new Stack<Integer>();
        while(l1!=null){ s1.push(l1.val);l1=l1.next;}
        while(l2!=null){ s2.push(l2.val);l2=l2.next;}
        int sum = 0;
        ListNode list = new ListNode(0);
        while(!s1.empty()||!s2.empty()){
            if(!s1.empty()) sum +=s1.pop();
            if(!s2.empty()) sum +=s2.pop();
            list.val = sum%10;
            ListNode head = new ListNode(sum/10);
            head.next = list;
            list = head;
            sum /=10;
        }
        return list.val==0 ? list.next : list;
    }
}