Add Two Numbers II


#1

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

#2

The below code uses a Stack to avoid reversing the list and adds them up.

class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        if(l1==null) return l2;
        else if(l2==null) return l1;
        Stack<ListNode> s1 = new Stack<>();
        Stack<ListNode> s2 = new Stack<>();
        ListNode t1=l1, t2=l2;
        while(t1!=null){ s1.push(t1); t1=t1.next;}
        while(t2!=null){ s2.push(t2); t2=t2.next;}
        ListNode res = new ListNode(0);
        int sum=0;
        while(!s1.isEmpty() || !s2.isEmpty()){
            if(!s1.isEmpty()){
                sum += s1.pop().val;
            }
            if(!s2.isEmpty()){
                sum += s2.pop().val;
            }
            ListNode temp = new ListNode(sum%10);
            sum /= 10;
            temp.next =res.next;
            res.next = temp;
        }
        if(sum!=0){
            ListNode temp = new ListNode(sum);
            temp.next =res.next;
            res.next = temp;
        }
        return res.next;
    }
}