Stack (C language)

typedef int STDataType;
typedef struct Stack
{
    
	STDataType* a;
	int top;
	int capacity;
}ST;

void StackInit(ST* ps)
{
    
	ps->a = NULL;
	ps->top = ps->capacity = 0;
}

void StackDestroy(ST* ps)
{
    
	free(ps->a);
	ps->a = NULL;
	ps->capacity = ps->top = 0;
}

void StackPush(ST* ps, STDataType x)
{
    
	if (ps->top == ps->capacity)
	{
    
		int newcapacity = ps->capacity == 0 ? 4 : ps->capacity * 2;
		STDataType* tmp = (STDataType*)realloc(ps->a,sizeof(STDataType) * newcapacity);
		ps->capacity = newcapacity;
		if (tmp == NULL)
		{
    
			exit(-1);
		}
		ps->a = tmp;
	}
	ps->a[ps->top++] = x;
}

void StackPop(ST* ps)
{
    
	if (StackEmpty(ps))
		return;
	ps->top--;
}

STDataType StackTop(ST* ps)
{
    
	return ps->a[ps->top-1];
}

bool StackEmpty(ST* ps)
{
    
	return ps->top == 0;
}

int StackSize(ST* ps)
{
    
	return ps->top;
}