Get record id from standard controller using Apex in Salesforce?

Sample Code:

Visualforce page:

<apex:page standardcontroller="account" extensions="SampleController">
{!accountid}
</apex:page>

Apex Controller:

public class SampleController {

    public Id accountid {get;set;}

  

    public Sample(ApexPages.StandardController controller) {

        Account acc = new Account();

        accountid = controller.getId();

    }       

}