新建一个Stack栈,用来对数组进行操作
@interface Stack : NSObject

- (NSInteger)pop;

- (void)push:(NSInteger)number;

- (BOOL)isOperator:(NSString *)str;

@property(nonatomic, strong)NSMutableArray *stack;

@end

@implementation Stack

- (instancetype)init
{
    self = [super init];
    if (self) {
        _stack = [NSMutableArray array];
    }
    return self;
}

- (NSInteger)pop
{
    if (_stack.count > 0){
        NSInteger lastNumber = ((NSNumber *)(_stack.lastObject)).integerValue;
        [_stack removeLastObject];
        return lastNumber;
    }
    
    return (long)0;
}

- (void)push:(NSInteger)number
{
    [_stack addObject:[[NSNumber alloc]initWithInteger:number]];
}

- (BOOL)isOperator:(NSString *)str{
    
    return [@"+-*/" containsString:str];
}

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view.
    
    //逆波兰算法
    NSArray *opArray = @[@"10", @"6", @"9", @"3", @"+", @"-11", @"*", @"/", @"*", @"17", @"+", @"5", @"+"];
    
    Stack *stack = [[Stack alloc] init];
    NSInteger result = 0;
    for (NSString *str in opArray)
    {
       if ([stack isOperator:str] == NO)
       {
           [stack push:str.integerValue];
       }else {
           NSInteger leftOperator = result == 0 ? [stack pop] : result;
           NSInteger rightOperator = [stack pop];
           if ([str isEqualToString:@"+"]){
               result = rightOperator+leftOperator;
           }else if ([str isEqualToString:@"-"]){
               result = rightOperator-leftOperator;
           }else if ([str isEqualToString:@"*"]){
               result = rightOperator*leftOperator;
           }else{
               result = rightOperator/leftOperator;
           }
       }
    }
    
    NSLog(@"");
    
    
}
@end