#!/usr/bin/env python
# coding: utf-8

# <h1>2. Add Two Numbers</h1>
# <hr>
# 
# <!--Copy Paste Leetcode statement between-->
# <p>You are given two <b>non-empty</b> linked lists representing two non-negative integers. The digits are stored in <b>reverse order</b>, and each of their nodes contains a single digit. Add the two numbers and return the sum&nbsp;as a linked list.</p>
# 
# <p>You may assume the two numbers do not contain any leading zero, except the number 0 itself.</p>
# 
# <p>&nbsp;</p>
# <p><strong>Example 1:</strong></p>
# <img alt="" src="./img1.jpeg" style="width: 483px; height: 342px;">
# <pre><strong>Input:</strong> l1 = [2,4,3], l2 = [5,6,4]
# <strong>Output:</strong> [7,0,8]
# <strong>Explanation:</strong> 342 + 465 = 807.
# </pre>
# <p>&nbsp;</p>
# <p><strong>Example 2:</strong></p>
# 
# <pre><strong>Input:</strong> l1 = [0], l2 = [0]
# <strong>Output:</strong> [0]
# </pre>
# 
# <p><strong>Example 3:</strong></p>
# 
# <pre><strong>Input:</strong> l1 = [9,9,9,9,9,9,9], l2 = [9,9,9,9]
# <strong>Output:</strong> [8,9,9,9,0,0,0,1]
# </pre>
# 
# <p>&nbsp;</p>
# <p><strong>Constraints:</strong></p>
# 
# <ul>
# 	<li>The number of nodes in each linked list is in the range <code>[1, 100]</code>.</li>
# 	<li><code>0 &lt;= Node.val &lt;= 9</code></li>
# 	<li>It is guaranteed that the list represents a number that does not have leading zeros.</li>
# </ul>
# <!--Copy Paste Leetcode statement between-->
# 
# <p>&nbsp;</p>
# <a href="https://leetcode.com/problems/add-two-numbers/">Source</a> 
# <hr>

# In[1]:


# Definition for singly-linked list.
class ListNode:
    def __init__(self, val=0, next=None):
        self.val = val
        self.next = next


# <h4>Code</h4>

# In[2]:


def add_two_numbers(l1, l2):
    """iterative approach."""
    carry = 0
    answer = curr_sol = ListNode(carry)
    while l1 or l2 or carry != 0:
        tmp = carry
        if l1:
            tmp += l1.val
            l1 = l1.next
        if l2:
            tmp += l2.val
            l2 = l2.next
        val = tmp % 10
        carry = tmp // 10
        curr_sol.next = ListNode(val)
        curr_sol = curr_sol.next
    return answer.next


# <hr>
# <h4>Follow up:</h4>
# <p>Solve it both recursively and iteratively.</p>
# 
# <h4>Code</h4>

# In[ ]:


def add_two_numbers(l1, l2):
    """recursive approach."""
    def helper(l1,l2,carry):
        if l1 or l2 or carry != 0:
            tmp = carry
            if l1:
                tmp += l1.val
                l1 = l1.next
            if l2:
                tmp += l2.val
                l2 = l2.next
            val = tmp % 10
            carry = tmp // 10
            answer = ListNode(val) 
            answer.next = helper(l1, l2, carry)    
            return answer
        return None

    return helper(l1,l2,0)