题目:

用两个栈实现一个队列。队列的声明如下，请实现它的两个函数appendTail 和deleteHead，分别完成在队列尾部插入结点和在队列头部删除结点的功能。

思路:

image.png image.png

解答:

@interface NSCustomQueue : NSObject

// 添加 到 尾部
- (void)appendTail:(id)object;

// 删除 头部
- (BOOL)deleteHeader;
@end

实现文件:

#import "NSCustomStack.h"
#import "NSCustomQueue.h"

@interface NSCustomQueue()
// 入栈 队列
@property (nonatomic, strong) NSCustomStack *pushStack;

// 出栈 队列
@property (nonatomic, strong) NSCustomStack *popStack;
@end

@implementation NSCustomQueue

#pragma mark --------------- Public Methods
// 添加 到 尾部
- (void)appendTail:(id)object {
    [self.pushStack push:object];
}

// 删除 头部
- (BOOL)deleteHeader {
    if ([self.popStack isEmpty]) {
        __weak typeof(self) weakSelf = self;
        [self.pushStack traversalElementPopStack:^(id objc) {
            [weakSelf.popStack push:objc];
        }];
    }
    
    if ([self.popStack isEmpty]) {
        return NO;
    }
    
    [self.popStack popTopElement];
    return YES;
}

- (void)printWithBlock:(NSCustomQueueBlock)block {
    if (self.popStack.stackLength > 0) {
        [self.popStack traversalElementFromtop:^(id objc) {
            if (block) {
                block(objc);
            }
        }];
    }
    
    if (self.pushStack.stackLength > 0) {
        [self.pushStack traversalElementFromBottom:^(id objc) {
            if (block) {
                block(objc);
            }
        }];
    }
}
#pragma mark --------------- Getter / Setter

// 入栈 队列
- (NSCustomStack *)pushStack {
    if(!_pushStack){
        _pushStack = [[NSCustomStack alloc] init];

    }
    return  _pushStack;
}


// 出栈 队列
- (NSCustomStack *)popStack {
    if(!_popStack){
        _popStack = [[NSCustomStack alloc] init];
    }
    return  _popStack;
}
@end

main函数:

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        
        NSCustomQueue *customQueue = [[NSCustomQueue alloc] init];
        [customQueue appendTail:@"1"];
        [customQueue appendTail:@"2"];
        [customQueue appendTail:@"3"];
        
        [customQueue deleteHeader];
        [customQueue appendTail:@"4"];
        [customQueue appendTail:@"5"];
        [customQueue printWithBlock:^(id objc) {
            NSLog(@"%@", (NSString *)objc);
        }];
    }
    return 0;
}