FLAG_ACTIVITY_CLEAR_TOP clears your Activity stack , you can use the code below:
Intent intent = new Intent(this, Activity.class);
intent.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
startActivity(intent);
Remember that this flag clears just Intermediate Activities , for example if you have A,B,C in your Back Stack then going from C Activity to D with this flag this does not clear Back Stack and the Stack would be A,B,C,D but if you go from Activity D to Activity A with this flag , B,C,D Activities will pop up from the stack and you will have just A in the Back Stack .