CC4.1.java

    public boolean isBalanced(Tree root) {
        if (checkHeight(root) == -1)
            return false;
        else
            return true;
    }
    
    private int checkHeight(Tree root) {
        if (root == null)
            return 0;
        int left_h = checkHeight(root.left);
        if (left_h == -1)
            return -1;
        int right_h = checkHeight(root.right);
        if (right_h == -1 || Math.abs(left_h - right_h) > 1)
            return -1;
        return Math.max(left_h, right_h) + 1;
    }