ctciCh4Q1

    public boolean isBalanced() {
        return checkHeight(this.root) == -1 ? false : true;
    }

    private int checkHeight(TreeNode node) {
        if(node == null){
            return 0;
        }
        int leftHeight = checkHeight(node.leftChild);
        if (leftHeight == -1) {
            return -1;
        }
        int rightHeight = checkHeight(node.rightChild);
        if (rightHeight == -1) {
            return -1;
        }

        if (Math.abs(leftHeight - rightHeight) > 1) {
            return -1;
        } else {
            return Math.max(leftHeight, rightHeight)+1;
        }
    }