count-good-nodes in bt: add py3 soln

1544_count-good-nodes-in-binary-tree/python3/solution.py

@@ -0,0 +1,37 @@
+# Time: O(N)
+# Space: O(N)
+
+# Definition for a binary tree node.
+# class TreeNode:
+#     def __init__(self, val=0, left=None, right=None):
+#         self.val = val
+#         self.left = left
+#         self.right = right
+class Solution:
+    def goodNodes(self, root: TreeNode) -> int:
+        def dfs(node, max_value) -> int:
+            '''
+            Perform pre-order traversal and keep track of max
+            elements in the tree. Any subsequent traversal can
+            then compare against the updated max_value to see if
+            it's a good node or not
+            '''
+            
+            # If no left/right nodes, then we can just return 0
+            if not node: return 0
+            
+            # Current node is good if it's value is greater than or
+            # equal to the `max_value` seen so far
+            res = 1 if max_value <= node.val else 0
+            
+            # Compute the new max value, the current node could be it
+            max_value = max(max_value, node.val)
+            
+            # Do traversal on left and right nodes and add their units
+            res += dfs(node.left, max_value) + dfs(node.right, max_value)
+            
+            # This will indicate the count
+            return res
+        
+        
+        return dfs(root, root.val)