CareerCup

use stacks

May be like this ,

interface Action { 
 execute() ;
 Action getInverseAction() ;
}

class DrawLineAction implements Action { 
   DrawLineAction (Point p1, Point p2) {
   }

    public execute() {
    // Logic to draw a line 
   }
  
   public getInverseAction() {
        return new EraseLineAction(p1,p2) ;
   }

} 

class EraseLineAction implements Action {
EraseLineAction (Point p1, Point p2) {
}

public execute() {
   //Logic to erase a line
}

public Action getInverseAction() {
    return new DrawLineAction(p1,p2);
}

-------------------------

Once you have this setup , you can maintain a linked list . And have the index as current Action . For every line you draw you will add the Action to the list

List<Action>  m_actions = new ... 
Action m_currentAction ; 

drawObject(...)  {
    for each point { 
         Point p1 , Point p2 ;
        Action act = new DrawLineActin(p1,p2) ;
        m_actions.append(act);
       act.execute() ;
       m_currentAction = act ;
    }
}

undo() {
Action act = m_currentAction.getInverseAction();
action.execute();
m_currentAction = (Previous Action in m_actions() )
}

redo() {
   action =  (next Action of m_currentAction in m_actions) 
   action.execute() ;
   m_currentAction = action ;
}

Store states of the progress in a stack.

Look for the Memento pattern for the undo-redo operations.