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0002-add-two-numbers/0002-add-two-numbers.py

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+# 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]:
+        """
+        Complexities:
+        Time:  O(n)
+        Space: O(1)
+
+        where n = max(len(l1), len(l2))
+        """
+
+        # Addition by place value
+        currNode1 = l1
+        currNode2 = l2
+        sumNode = None
+        tailNode = None
+        carry = 0
+        placeValue = 1
+
+        # Loop through nodes until reached end of both lists
+        while currNode1 or currNode2:
+            v1 = currNode1.val if currNode1 else 0 
+            v2 = currNode2.val if currNode2 else 0 
+
+            sumVal = (v1 + v2 + carry) * placeValue
+            val = sumVal % 10
+            carry = sumVal // 10
+            newNode = ListNode(val)
+            
+            if sumNode == None:
+                sumNode = newNode
+            else:
+                tailNode.next = newNode
+
+            tailNode = newNode
+
+            currNode1 = currNode1.next if currNode1 else None
+            currNode2 = currNode2.next if currNode2 else None
+
+        if carry:
+            tailNode.next = ListNode(carry)
+
+        return sumNode or ListNode(0)
+        
+