1. a) The postorder traversal of a Binary Search Tree (BST) is given below. 40 80 10 15 25 120 130 150 110 100 Draw the BST and briefly explain the process you followed to build the tree. b) Write a function f1 that takes the root of a binary tree as a parameter and returns the sum of the nodes which are in the left subtree of the root. You must not consider the value of the root in the final sum. Consider the following class definitions while writing your code. class Node { public: int key; }; Node left; Node right;