i used the inbuilt stack class . but its doesnt showing the right answer . is there something wrong with my approach ??
May you please explain what's wrong with my code
There is no problem in inbuilt stack. The problem is in the display function
Your i loop in display() is from 0 to primary.size(). But the size of primary stack is changing everytime you are popping an element from it. For example, after enqueueing the 0,1,2,3,4 in the primary stack, you pop 0 and print 0. Then size reduces to 4. Now it’ll print 1 and pop it so the size reduces to 3 (i.e. 2,3,4). Now it will run and pop and print 2 which leaves the primary stack as [3, 4] of size 2 but your i (loop wala) is incremented to 3 which is greater than primary.size(). So it comes out of that loop.
The required change is easy i hope you get it. If not let me know! 
thanx , it’s resolved now