Added flatten-tree-to-ll

Author: avidLearnerInProgress

flatten_binary_tree.py

@@ -0,0 +1,27 @@
+# Definition for a binary tree node.
+# class TreeNode:
+#     def __init__(self, x):
+#         self.val = x
+#         self.left = None
+#         self.right = None
+
+class Solution:
+    def __init__(self):
+        self.prev = None
+        
+    def flatten(self, root: TreeNode) -> None:
+        """
+        Do not return anything, modify root in-place instead.
+        #reverse postorder traversal
+        
+        """
+        if not root: return None
+    
+        self.flatten(root.right)
+        self.flatten(root.left)
+        
+        root.right = self.prev
+        root.left = None
+        self.prev = root
+        
+