Added expression_tree_evaluation.py file

Added expression_tree_evaluation.py file which contains code in python for the same.

Author: pushpit-J19

Data Structures/Trees/expression_tree_evaluation.py

@@ -0,0 +1,36 @@
+class Node :
+    """Class to define tree nodes, 
+        this has a value attribute to store the value, and 
+        left and right attributes to point to the left and right child respectively"""
+    def __init__(self, value):
+        self.value = value
+        self.right = None
+        self.left = None
+    
+def expression_tree_evaluation (node):
+    """A recursive function which evaluates the expression subtree passed to it
+        and returns the evaluated value"""
+    if node == None:    # Node doesn't exist
+        return 0
+    if node.left == None and node.right == None:    # leaf node containing an operand
+        return node.value
+    
+    # node is an operator
+    left_child = expression_tree_evaluation(node.left)
+    right_child = expression_tree_evaluation (node.right)
+
+    return eval(str(left_child)+node.value+str(right_child))
+
+
+# DRIVER CODE
+
+if __name__ == '__main__':
+    # Expression in the example : (5*4)+(100-2)
+    root = Node('+')
+    root.left = Node('*')
+    root.left.left = Node('5')
+    root.left.right = Node('4')
+    root.right = Node('-')
+    root.right.left = Node('100')
+    root.right.right = Node('20')
+    print (expression_tree_evaluation(root))