Using Assembly language( Using Accumulator)...Without using 
any arithmatic...without using any Pointer...without 
declaring third varible

int a = 20;
int b = 10;
   __asm
  {
	  mov EAX,b
	  push EAX
	  mov EAX,a
	  mov b,EAX
	  pop EAX
	  mov a,EAX
  }