iOS 实现一个栈
苹果的Cocoa并没有暴露系统的栈结构 ，这里根据栈的特点，使用数组实现了一个简单的栈。

MyStack.h

@interface MyStack : NSObject

- (void)push:(id)obj;

- (id)pop;

- (id)peek;

@end

MyStack.m

#import "MyStack.h"

// 定义栈的容量
#define kStackSize  10

@interface MyStack ()

@property (nonatomic, strong) NSMutableArray *list;

@property (nonatomic, assign) NSInteger size; // the number of stack elements

@end

@implementation MyStack

- (instancetype)init
{
    self = [super init];
    if (self) {
        
        self.list = [NSMutableArray arrayWithCapacity:kStackSize];
        self.size = 0;
        
    }
    return self;
}

- (void)push:(id)obj
{
    if ([self isFull] || !obj) return;

    [self.list addObject:obj];
    self.size++;
}

- (id)pop
{
    if ([self isEmpty]) return nil;
    
    id obj = [self.list objectAtIndex:self.size -1];
    [self.list removeObjectAtIndex:self.size - 1];
    self.size--;
    return obj;
}

- (id)peek
{
    if ([self isEmpty]) return nil;
    
    id obj = [self.list objectAtIndex:self.size -1];
    return obj;
}

- (BOOL)isEmpty
{
    return self.size == 0;
}

- (BOOL)isFull
{
    return self.size == kStackSize;
}
@end

线程安全的 MyStack.m

#import "MyStack.h"

#define kStackSize  10

@interface MyStack ()

@property (nonatomic, strong) NSMutableArray *list;

@property (nonatomic, assign) NSInteger size; // the number of stack elements

@property (nonatomic, strong) dispatch_semaphore_t semaphore;  // 信号量  锁

@end


@implementation MyStack

- (instancetype)init
{
    self = [super init];
    if (self) {
        
        self.list = [NSMutableArray arrayWithCapacity:kStackSize];
        self.size = 0;
        self.semaphore = dispatch_semaphore_create(1);
        
    }
    return self;
}

- (void)push:(id)obj
{
    if ([self isFull] || !obj) return;
    
    dispatch_semaphore_wait(self.semaphore, DISPATCH_TIME_FOREVER);
    [self.list addObject:obj];
    self.size++;
    dispatch_semaphore_signal(self.semaphore);
}

- (id)pop
{
    if ([self isEmpty]) return nil;
    
    dispatch_semaphore_wait(self.semaphore, DISPATCH_TIME_FOREVER);
    id obj = [self.list objectAtIndex:self.size -1];
    [self.list removeObjectAtIndex:self.size - 1];
    self.size--;
    dispatch_semaphore_signal(self.semaphore);
    return obj;
}

- (id)peek
{
    if ([self isEmpty]) return nil;
    
    id obj = [self.list objectAtIndex:self.size -1];
    return obj;
}

- (BOOL)isEmpty
{
    return self.size == 0;
}

- (BOOL)isFull
{
    return self.size == kStackSize;
}

@end

测试代码

MyStack *stack = [[MyStack alloc] init];

[stack push:@(1)];
[stack push:@(2)];
[stack push:@(3)];

NSLog(@"%@", [stack peek]);

NSLog(@"%@", [stack pop]);
NSLog(@"%@", [stack pop]);
NSLog(@"%@", [stack pop]);
NSLog(@"%@", [stack pop]);


输出结果：
2022-01-02 21:52:04.782525+0800 MyStack[15061:347654] 3
2022-01-02 21:52:04.783088+0800 MyStack[15061:347654] 3
2022-01-02 21:52:04.783163+0800 MyStack[15061:347654] 2
2022-01-02 21:52:04.783226+0800 MyStack[15061:347654] 1
2022-01-02 21:52:04.783247+0800 MyStack[15061:347654] (null)