Create 002_add_two_numbers.md

Author: Joseph Luce

leetcode/medium/002_add_two_numbers.md

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+# 2. Add Two Numbers
+
+## Best Solution
+- Runtime: O(L1) + O(L2)
+- Space: O(L1) + O(L2)
+- L1 - Nodes in list 1
+- L2 - Nodes in list 2
+
+The only tricky part is the carry over.
+You could implement it with a boolean for the carry over, however, I believe it creates more if else statements.
+Instead, if you used a rolling sum instead and removed the last digit of the sum, it will be more elegant.
+
+```
+class Solution:
+    def __init__(self):
+        self._sum = 0
+        self._dummy_head = ListNode(-1)
+        self._tail = self._dummy_head
+    
+    def addTwoNumbers(self, l1: ListNode, l2: ListNode) -> ListNode:
+        while l1 and l2:
+            digit = self.get_digit(l1.val+l2.val)
+            self.add_new_node(digit)
+            l1, l2 = l1.next, l2.next
+        while l1:
+            digit = self.get_digit(l1.val)
+            self.add_new_node(digit)
+            l1 = l1.next
+        while l2: 
+            digit = self.get_digit(l2.val)
+            self.add_new_node(digit)
+            l2 = l2.next
+        if self._sum != 0:
+            self.add_new_node(1)
+        return self._dummy_head.next
+            
+    def get_digit(self, _sum):
+        self._sum += _sum
+        digit = self._sum % 10
+        self._sum //= 10
+        return digit
+    
+    def add_new_node(self, digit):
+        new_node = ListNode(digit)
+        self._tail.next = new_node
+        self._tail = new_node
+```