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

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

@@ -0,0 +1,39 @@
+# 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