#include <stdio.h>
#include <stdlib.h>
#include <string.h>

struct node {
    int idata;
    struct node* pnext;

};

char *fetchline(char *s)
{
    /* get one line from standard input */
	register int c;
	int n = 80;
	register char *pcs;
	pcs = s;
	while (--n > 0 && (c = getchar()) != EOF)
		if ((*pcs++ = c) == '\n')
			break;
	*pcs = '\0';
	return (c == EOF && pcs == s) ? NULL : s;

}

int ipop(struct node ** ppnodeHead, int *piPopped)
{
	/* pops the first value off the stack */
	/*TO BE IMPLEMENTED */
	*piPopped = 0;   /* the value popped from the stack */
	return 0;        /* if successful return 0 otherwise 1 */

}

void pop(struct node ** ppnodeHead)
{
    /* pops and then prints the value on top of the stack */
	int iPopped, iResult;
	iResult = ipop(ppnodeHead, &iPopped);
	if (iResult == 1)
	{
		printf("NULL\n");
	}
	else
	{
		printf("%d\n", iPopped);
	}
}

void clear(struct node ** ppnodeHead)
{
    /* delete everything from the stack */
    printf("TO BE IMPLEMENTED\n");
	
}

void peek(struct node * pnodeHead)
{
    /* prints the top value of the stack. */
    printf("TO BE IMPLEMENTED\n");
}

void printstack(struct node * pnodeHead)
{
    /* prints the entire stack top down. */
    printf("TO BE IMPLEMENTED\n");
}

void push(struct node ** ppnodeHead, int iarg)
{
	/* push value iarg onto stack */
	printf("TO BE IMPLEMENTED\n");
}

void add(struct node ** ppnodeHead)
{
    /* pops 2 values off stack; adds them; and then pushes result
    back on stack. It prints nothing */
	printf("TO BE IMPLEMENTED\n");
}

void subtract(struct node ** ppnodeHead)
{
    /* pops 2 values off stack; Subtracts the first number popped from
       the second and then pushes result back on stack. It prints nothing */
	printf("TO BE IMPLEMENTED\n");
}



int main(int argc, char *argv[])
{
	struct node * pnodeHead = NULL;
	/*struct node * pnodeTail = NULL; */
	char cline[81];
    char pcOp[20];
    int iarg;
	
    while(fetchline(cline))
    {
       if (sscanf(cline, "%s", pcOp))
       {
           if (strcmp(pcOp, "PUSH") == 0)
           {
           		if (sscanf(cline, "%s %d", pcOp, &iarg))
				{
           			push(&pnodeHead, iarg);
           		}
           		
           }
           else if (strcmp(pcOp, "POP") == 0)
		   {
		   	   	pop(&pnodeHead);
		   }
		   else if (strcmp(pcOp, "ADD") == 0)
		   {
		   		add(&pnodeHead);
		   }
		   
		   else if (strcmp(pcOp, "SUBTRACT") == 0)
		   {		
		   	    subtract(&pnodeHead);
		   }
		   else if (strcmp(pcOp, "PEEK") == 0)
		   {		
		   	    peek(pnodeHead);
		   }
		   else if (strcmp(pcOp, "PRINTSTACK") == 0)
		   {
		   		printstack(pnodeHead);
		   }
		   else if (strcmp(pcOp, "CLEAR") == 0)
		   {
		   		clear(&pnodeHead);
		   }
		}
    }
    return 0;
}