gistfile1.java

//in Java
public boolean isValidBST(TreeNode root) {
    if(root == null) return true;
    
    return isValidBSTRec(root, Integer.MIN_VALUE, Integer.MAX_VALUE);
}

public boolean isValidBSTRec(TreeNode root, int min, int max){
    if(root == null) return true;
    
    if(root.val>min && root.val<max && isValidBSTRec(root.left, min, root.val)&&isValidBSTRec(root.right, root.val, max))
        return true;
    else 
        return false;
}