check if a tree is a binary search tree (BST)

isBST(a)
{
if(!a)
return true

if( a->left && minVal(a->left) > a->num)
return false

if( a->right && maxVal(a->right) < a->num)
return false

if(!isBST(a->left) && !isBST(a->right))
return false

return true

}