<apex:page controller="SampleTest" sidebar="false">
    <apex:form >
        <apex:pageBlock title="Account Table">
            <apex:pageBlockTable value="{!Acclst}" var="A">
            <apex:column headerValue="Account Name" > 
            <apex:commandLink value="{!A.Name}" action="{!showContacts}" rerender="Accwithcon">
              <apex:param value="{!A.Id}" name="idForConts" assignTo="{!recid}"/>
            </apex:commandLink>
            </apex:column>  
            <apex:column value="{!A.Id}"/>
            </apex:pageBlockTable>
        </apex:pageBlock> 
    
    <apex:pageBlock title="Contacts" id="Accwithcon" >
      <apex:pageBlockTable value="{!contacts}" var="contact" >
          <apex:column value="{!contact.id}"/>
          <apex:column value="{!contact.Lastname}"/>
          <apex:column value="{!contact.firstname}"/>
      </apex:pageBlockTable>
    </apex:pageBlock>
    </apex:form>
</apex:page>

public class SampleTest {
    
	 public string recid{get;set;}
     public string reccon{get;set;}
    public boolean flag{get;set;}

   public string row{ get; set;}
   public list<Account> Acclst{get;set;}

   public List<Contact> contacts {get; set;}

   public Nov25Task(){

   Acclst = [select Id,Name from Account limit 10];
   contacts=null;
       flag=false;
   }

  public void showContacts()
  {
     contacts=[select id, FirstName, LastName from Contact where AccountId=:recId];
      }
}