// Decision tree structure
    const loanDecisionTree = new BinaryTree<string>(
      ['stableIncome', 'goodCredit', 'Rejected', 'Approved', 'Rejected'],
      { isDuplicate: true }
    );

    function determineLoanApproval(
      node?: BinaryTreeNode<string> | null,
      conditions?: { [key: string]: boolean }
    ): string {
      if (!node) throw new Error('Invalid node');

      // If it's a leaf node, return the decision result
      if (!node.left && !node.right) return node.key;

      // Check if a valid condition exists for the current node's key
      return conditions?.[node.key]
        ? determineLoanApproval(node.left, conditions)
        : determineLoanApproval(node.right, conditions);
    }

    // Test case 1: Stable income and good credit score
    console.log(determineLoanApproval(loanDecisionTree.root, { stableIncome: true, goodCredit: true })); // 'Approved'

    // Test case 2: Stable income but poor credit score
    console.log(determineLoanApproval(loanDecisionTree.root, { stableIncome: true, goodCredit: false })); // 'Rejected'

    // Test case 3: No stable income
    console.log(determineLoanApproval(loanDecisionTree.root, { stableIncome: false, goodCredit: true })); // 'Rejected'

    // Test case 4: No stable income and poor credit score
    console.log(determineLoanApproval(loanDecisionTree.root, { stableIncome: false, goodCredit: false })); // 'Rejected'

    const expressionTree = new BinaryTree<number | string>(['+', 3, '*', null, null, 5, '-', null, null, 2, 8]);

    function evaluate(node?: BinaryTreeNode<number | string> | null): number {
      if (!node) return 0;

      if (typeof node.key === 'number') return node.key;

      const leftValue = evaluate(node.left); // Evaluate the left subtree
      const rightValue = evaluate(node.right); // Evaluate the right subtree

      // Perform the operation based on the current node's operator
      switch (node.key) {
        case '+':
          return leftValue + rightValue;
        case '-':
          return leftValue - rightValue;
        case '*':
          return leftValue * rightValue;
        case '/':
          return rightValue !== 0 ? leftValue / rightValue : 0; // Handle division by zero
        default:
          throw new Error(`Unsupported operator: ${node.key}`);
      }
    }

    console.log(evaluate(expressionTree.root)); // -27