@RestResource(urlMapping='/CreateCustomer/*')
global with sharing class CreateCustomerRestAPI {

    @HttpPost
    global static string doPost(){
        RestRequest  req     = RestContext.request;   
         String jsonInput  = req.requestBody.toString();


        AccountWrapper d = (AccountWrapper)JSON.deserialize(jsonInput, AccountWrapper.class);

    return null;

}   

    public class AccountWrapper{
    public String name; //AccountTest
    public cls_contact contact;
    public class cls_contact {
        public String firstName; 
        public String secondName;
    }
  }

}

@RestResource(urlMapping='/CreateCustomer/*')
global with sharing class CreateCustomerRestAPI {

    @HttpPost
    global static string doPost(){
        RestRequest  req     = RestContext.request;   
         String jsonInput  = req.requestBody.toString();


        AccountWrapper d = (AccountWrapper)JSON.deserialize(jsonInput, AccountWrapper.class);

    return null;

}   

    public class AccountWrapper{
    public String name; //AccountTest
    public cls_contact contact;
    public class cls_contact {
        public String firstName; 
        public String secondName;
    }
  }

}