quick update 235

Author: Yu Zhou

README.md

@@ -109,7 +109,7 @@
 |270|[Closest Binary Search Tree Value](https://leetcode.com/problems/closest-binary-search-tree-value/#/description)| [Python ](./tree/Yu/270.py)| _O(N)_| _O(1)_|[:tv:](https://youtu.be/RwvQyKJxHZk)|
 |563|[Binary Tree Tilt](https://leetcode.com/problems/binary-tree-tilt/#/description)| [Python ](./tree/Yu/563.py) | _O(N)_| _O(1)_ |[:tv:](https://youtu.be/47FQVP4ynk0)|
 |538|[Convert BST to Greater Tree](https://leetcode.com/problems/convert-bst-to-greater-tree/#/description)| [Python ](./tree/Yu/538.py) | _O(N)_| _O(1)_ |[:tv:](https://youtu.be/YoEPWtXmkD4)|
-
+|637|[Average of Levels in Binary Tree](https://leetcode.com/problems/average-of-levels-in-binary-tree/)| [Python ](./tree/Yu/637.py) | _O(N)_| _O(1)_ ||
 
 ## Tree Medium
 |  #  | Title | Solution | Time | Space | Video|

tree/Yu/637.py

@@ -0,0 +1,33 @@
+class Solution(object):
+    # DFS Recursion
+    def averageOfLevels(self, root):
+        self.res = []
+
+        def dfs(root, level):
+            if not root:
+                return []
+            if len(self.res) == level:
+                self.res.append([])
+            self.res[level].append(root.val)
+            dfs(root.left, level + 1)
+            dfs(root.right, level + 1)
+
+        dfs(root, 0)
+        return [sum(num)/float(len(num)) for num in self.res]
+
+    # DFS Recursion, Optimized using Tuples
+    def averageOfLevels2(self, root):
+        self.res = []
+
+        def dfs(root, level):
+            if not root:
+                return []
+            if len(self.res) == level:
+                self.res.append([0,0])
+            self.res[level][0] += root.val
+            self.res[level][1] += 1.0
+            dfs(root.left, level + 1)
+            dfs(root.right, level + 1)
+
+        dfs(root, 0)
+        return [num/count for num, count in self.res]