isValidBST.java

private boolean isValidBSTHelper(TreeNode root, int min, int max) {
    if(root == null)
        return true;

    if(root.val < min || root.val > max)
        return false;

    if(root.val == Integer.MIN_VALUE && root.left != null)
        return false;
    if(root.val == Integer.MAX_VALUE && root.right != null)
        return false;

    return  isValidBSTHelper(root.left, min, root.val - 1) &&
            isValidBSTHelper(root.right, root.val + 1, max);
}

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

root.val < min and not root.val <= min because it will give wrong output if the tree is [Integer.MIN_VALUE]