Verify a binary tree

This problem killed me once. the tricky thing is that only judging parent node greater than left child and less than right child is NOT enough. you need to make sure that the right most leave of your right subtree is also less than current node. A fake example is
  10
/
5
\
  12

	public boolean isBST() {
		return isBST(root, Integer.MIN_VALUE, Integer.MAX_VALUE);
	}

	private boolean isBST(TreeNode node, int minValue, int maxValue) {
		if (node == null) return true;
		if (node.data < minValue || node.data > maxValue) {
			return false;
		}
		boolean isBst = true;
		if (node.left != null) {
			isBst &= isBST(node.left, minValue, node.data);
			if (!isBst)
				return false;
		}
		if (node.right != null) {
			isBst &= isBST(node.right, node.data, maxValue);
			if (!isBst)
				return false;
		}
		return true;
	}