Objective-C中的堆栈实现

我在Objective-C中实现一个堆栈，当我测试我的课程时，我得到了无效的结果。以下是代码：

类/Stack.h：

#import <Foundation/Foundation.h>

#define DEFAULT_SIZE 16

@interface Stack : NSObject {
    @private int size;
    @private int top;
    @private NSMutableArray* arr;
}

- (id) init;
- (id) initWithSize: (int) size;

- (void) push: (id) element;
- (id)   pop;

- (id) peek;

- (int) size;

- (BOOL) isEmpty;

@end

类/Stack.m：

#import "Stack.h"

@implementation Stack

- (id) init {
    self = [super init];

    if(self) {
        size = DEFAULT_SIZE;
        top = 0;
        arr = [[NSMutableArray alloc] init];
    }

    return self;
}

- (id) initWithSize: (int) aSize {
    self = [super init];

    if(self) {
        if(aSize <= 0)
            @throw [NSException exceptionWithName:@"Invalid Argument" reason:@"Size should be strictly positive." userInfo:nil];

        size = aSize;
        top = 0;
        arr = [[NSMutableArray alloc] init];
    }

    return self;
}

- (void) push: (id) element {
    if(top > size)
        @throw [NSException exceptionWithName:@"Illegal State" reason:@"Stack contents exceed the valid limit." userInfo:nil];

    if(top == size)
        @throw [NSException exceptionWithName:@"Stack Overflow" reason:@"Stack is full, can not push any element." userInfo:nil];

    [arr addObject:element];
    top++;
}

- (id) pop {
    if(top < 0)
        @throw [NSException exceptionWithName:@"Illegal State" reason:@"Stack lower limit is invalid." userInfo:nil];

    if(top == 0)
        @throw [NSException exceptionWithName:@"Stack Underflow" reason:@"Stack is empty, can not pop any element" userInfo:nil];

//    id ret = [arr lastObject];
//    [arr removeLastObject];
//    top--;

    id ret = [arr objectAtIndex:(--top)];

    return ret;
}

- (id) peek {
    if(top < 0)
        @throw [NSException exceptionWithName:@"Illegal State" reason:@"Stack lower limit is invalid." userInfo:nil];

    if(top == 0)
        @throw [NSException exceptionWithName:@"Stack Underflow" reason:@"Stack is empty, can not pop any element" userInfo:nil];

//    return [arr lastObject];
    return [arr objectAtIndex:(top - 1)];
}

- (int) size {
    return top;
}

- (BOOL) isEmpty {
    return top == 0;
}

@end

和main.m：

#import <Foundation/Foundation.h>

#import "Classes/Stack.h"

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        Stack* stack = [[Stack alloc] initWithSize:3];

        [stack push:@"world"];
        [stack push:@" "];
        [stack push:@"hello"];


        for(int i = 0; i < [stack size]; i++)
            NSLog(@"%@", [stack pop]);
    }

    return 0;
}

我得到的结果是：

2014-01-07 00:09:36.045 Data Structures[9210:303] hello
2014-01-07 00:09:36.047 Data Structures[9210:303]  
Program ended with exit code: 0

我的代码有什么问题并产生无效的结果吗？