feat(0002_add-two-numbers): add py3 solution

0002_add-two-numbers/README.md

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+You are given two **non-empty** linked lists representing two non-negative integers. The digits are stored in **reverse order**, and each of their nodes contains a single digit. Add the two numbers and return the sum as a linked list.
+
+You may assume the two numbers do not contain any leading zero, except the number 0 itself.
+
+**Example 1:**
+
+![](https://assets.leetcode.com/uploads/2020/10/02/addtwonumber1.jpg)
+
+    Input: l1 = [2,4,3], l2 = [5,6,4]
+    Output: [7,0,8]
+    Explanation: 342 + 465 = 807.
+    
+
+**Example 2:**
+
+    Input: l1 = [0], l2 = [0]
+    Output: [0]
+    
+
+**Example 3:**
+
+    Input: l1 = [9,9,9,9,9,9,9], l2 = [9,9,9,9]
+    Output: [8,9,9,9,0,0,0,1]
+    
+
+**Constraints:**
+
+*   The number of nodes in each linked list is in the range `[1, 100]`.
+*   `0 <= Node.val <= 9`
+*   It is guaranteed that the list represents a number that does not have leading zeros.

0002_add-two-numbers/python3/solution.py

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+# Time:  O(max(m, n))
+# Space: O(max(m, n) + 1)
+
+# Definition for singly-linked list.
+# class ListNode:
+#     def __init__(self, val=0, next=None):
+#         self.val = val
+#         self.next = next
+class Solution:
+    def addTwoNumbers(self, l1: Optional[ListNode], l2: Optional[ListNode]) -> Optional[ListNode]:
+        sum_head = None
+        
+        l1ptr, l2ptr, curr = l1, l2, None
+        carry = 0
+        
+        # Iterate until l1 and l2 are exhausted
+        while l1ptr != None or l2ptr != None:
+            l1val = 0 if l1ptr is None else l1ptr.val
+            l2val = 0 if l2ptr is None else l2ptr.val
+            
+            # Basic sum with carry on the digits
+            raw_sum = l1val + l2val + carry
+            sum_wo_carry = raw_sum % 10
+            carry = 1 if raw_sum >= 10 else 0
+            
+            if sum_head is None:
+                curr = ListNode(sum_wo_carry)
+                sum_head = curr
+            else:
+                curr.next = ListNode(sum_wo_carry)
+                curr = curr.next
+            
+            l1ptr = None if l1ptr is None else l1ptr.next
+            l2ptr = None if l2ptr is None else l2ptr.next
+
+        if carry > 0:
+            curr.next = ListNode(carry)
+        
+        return sum_head