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

Solution1：stack

Time Complexity: O(N) Space Complexity: O(N)

Solution2：recursive

要先计算出length，否则在递归中不好同步
Time Complexity: O(N) Space Complexity: O(N)

Solution1 Code：

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

Solution2 Code：

class Solution2 {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        int size1 = getLength(l1);
        int size2 = getLength(l2);
        ListNode head = new ListNode(1);
        // Make sure l1.length >= l2.length
        head.next = size1 < size2 ? helper(l2, l1, size2 - size1) : helper(l1, l2, size1 - size2);
        // Handle the first digit
        if (head.next.val > 9) {
            head.next.val = head.next.val % 10;
            return head;
        }
        return head.next;
    }
    
    // offset is the difference of length between l1 and l2
    public ListNode helper(ListNode l1, ListNode l2, int offset) {
        if (l1 == null) return null;
        // check whether l1 becomes the same length as l2
        ListNode result = offset == 0 ? new ListNode(l1.val + l2.val) : new ListNode(l1.val);
        ListNode post = offset == 0 ? helper(l1.next, l2.next, 0) : helper(l1.next, l2, offset - 1);
        
        // handle carry 
        int c = 0;
        if(post != null) {
            c = post.val / 10;
            post.val = post.val % 10;
        }
        result.val += c;

        // combine nodes
        result.next = post;
        return result;
    }
    
    // get length of the list
    public int getLength(ListNode l) {
        int count = 0;
        while(l != null) {
            l = l.next;
            count++;
        }
        return count;
    }
}