Time limit: 5000ms
Memory limit: 256mb

Description:
Please complete the missing code of the stack ADT with an array, such that the program could output a correct answer.

-------------------------Copy the following code, complete it and submit-------------------------
/*
I, <Your Full Name>, am submitting the assignment for
an individual project.
I declare that the assignment here submitted is original except for
source material explicitly acknowledged, the piece of work, or a part
of the piece of work has not been submitted for more than one purpose
(i.e. to satisfy the requirements in two different courses) without
declaration. I also acknowledge that I am aware of University policy
and regulations on honesty in academic work, and of the disciplinary
guidelines and procedures applicable to breaches of such policy and
regulations, as contained in the University website
http://www.cuhk.edu.hk/policy/academichonesty/.
It is also understood that assignments without a properly signed
declaration by the student concerned will not be graded by the
teacher(s).
*/

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

typedef enum {PUSH = 1, POP, END, ERROR_OP} OP;

/* the minimum value of interger type. 0x80000000 = -2147483648
 * the maximum value of integer type. 0x7fffffff = 2147483647
 * They are used to represent the error of stack underflow and stack overflow, respectively
 */
#define ERROR_UNDERFLOW 0x80000000
#define ERROR_OVERFLOW  0x7fffffff

struct stack
{
    int *arr;
    int capacity;
    int top;
};

typedef struct stack Stack;

Stack *create_stack(int capacity)
{
    Stack *S = (Stack *)malloc(sizeof(struct stack));
    S->arr = (int*)malloc(sizeof(int) * capacity);
    S->capacity = capacity;
    S->top = 0;
    return S;
}

void destory_stack(Stack *S)
{
    free(S->arr);
    free(S);
    return ;
}

int is_full(Stack *S)
{
    // write down your code here
}

int is_empty(Stack *S)
{
    // write down your code here
}

int push(Stack *S, int x)
{
    if (is_full(S))
    {
        return ERROR_OVERFLOW;
    }

    // write down your code here

}

int pop(Stack *S)
{
    if (is_empty(S))
    {
        return ERROR_UNDERFLOW;
    }

    // write down your code here
}

OP get_op()
{
    char str[20];
    scanf("%s", str);
    if (strcmp(str, "push") == 0)
    {
        return PUSH;
    }
    else if (strcmp(str, "pop") == 0)
    {
        return POP;
    }
    else if (strcmp(str, "end") == 0)
    {
        return END;
    }
    else
    {
        return ERROR_OP;
    }
}

void print_array(Stack *S)
{
    int i;
    if (is_empty(S))
    {
        printf("empty\n");
        return ;
    }

    while (S->top > 0)
    {
        S->top -= 1;
        int x = S->arr[S->top];
        printf("%d ", x);
    }

    printf("\n");
    return ;
}

int main(void)
{
    int x;
    int capacity;
    Stack *S;
    int flag = 0;
    int ret = 0;

    scanf("%d", &capacity);
    S = create_stack(capacity);
    while (!flag)
    {
        switch(get_op())
        {
            case PUSH:
                scanf("%d", &x);
                ret = push(S, x);
                if (ret == ERROR_OVERFLOW)
                {
                    printf("stack overflow\n");
                }
                break;
            case POP:
                ret = pop(S);
                if (ret == ERROR_UNDERFLOW)
                {
                    printf("stack underflow\n");
                }
                else
                {
                    printf("%d\n", ret);
                }
                break;
            case END:
                print_array(S);
                flag = 1;
                break;
            default:
                printf("error input\n");
        }
    }

    destory_stack(S);
    return 0;
}

-------------------------------------------end of Code-------------------------------------------

Input:
The first line is a positive integer N, which denotes the capacity of the stack, followed by M lines of stack operations, each chosen from one of the followings: push x, pop.
The last line is an ending indicator, “end”.
Output:
The top element for each pop operation. If there exists underflow or overflow, please output "stack underflow" or "stack overflow", respectively.
The remaining elements in the stack from top to bottom, separated by a space. If the stack is empty, please output “empty”.

Sample Input 1:
4
pop
push 1
push 2
end

Sample Output 1:
stack underflow
2 1


Sample Input 2:
1
push 1
push 2
pop
end

Sample Output 2:
stack overflow
1
empty