#include <assert.h>

class Istack {
	int sz;
	int top;
	int* stack;
public:
	// Note that the stack is implicit
	void init(int nitems = 100) { sz = nitems; stack = new int[sz]; top = sz; }
	void push(int x) { assert( !isfull() ); stack[--top] = x; }
	int pop() { assert( !isempty() ); return( stack[top++] ); }
	bool isempty() { return( top == sz ); }
	bool isfull() { return( top == 0); }
};