Create 144_binary_tree_preorder_traversal.md

Author: Joseph Luce

leetcode/medium/144_binary_tree_preorder_traversal.md

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+# 144. Binary Tree Preorder Traversal
+
+## Recursive Solution
+- Runtime: O(N)
+- Space: O(N)
+- N = Number of elements in tree
+
+Preorder traversal is node -> left -> right
+
+```
+class Solution:
+    def preorderTraversal(self, root: TreeNode) -> List[int]:
+        def preorder_traversal_helper(root, result):
+            if root is None:
+                return
+            result.append(root.val)
+            preorder_traversal_helper(root.left, result)
+            preorder_traversal_helper(root.right, result)
+            
+        result = list()
+        preorder_traversal_helper(root, result)
+        return result
+```
+
+## Iterative Solution
+- Runtime: O(N)
+- Space: O(N)
+- N = Number of elements in tree
+
+This is one of the more easier iterative solutions out of the three traversals to understand.
+The current first in last out attributes of the stack conforms to the preorder traversal naturally.
+
+```
+class Solution:
+    def preorderTraversal(self, root: TreeNode) -> List[int]:
+        stack = list([root])
+        result = list()
+        while len(stack) > 0:
+            node = stack.pop()
+            if node is not None:
+                stack.append(node.right)
+                stack.append(node.left)
+                result.append(node.val)
+        return result
+```