Yi's Blog


Reverse a char

I met this problem in one interview: How to reverse a char? It took me a long time to figure out which I thought was very easy.

#include <iostream>
using namespace std;

unsigned char reverse_char(unsigned char in) {
    unsigned char high = 0x80;
    unsigned char res = 0x0, tmp = 0x0;
    for (int i=7; i>=0; i--) {
        tmp = (in&0x1)<<i;
        in = in>>1;
        res = (res&~high)|tmp;
        high = high>>1;
    return res;

int main(int argc, char *argv[]) {    
    printf("%x\n", reverse_char(0xc4));

You have to pay attention to the difference of char and unsigned char.

More info about signed char: What does it mean for a char to be signed? - Stack Overflow

- EOF -

Fix Soft Links in an Archived File

OS X 10.8 sdk was not included in Xcode 6.1, so if you want to use that sdk, you have to add it to /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/ yourself. For me, the reason I needed 10.8 sdk was I wanted to compile objc runtime source code with it so I can debug the code.

How to get the previous sdks

You can extract previous sdks from old versions fo Xcode. Or you can get them more efficently from this repository(phracker/MacOSX-SDKs) by using git or just downloading the zip file.

But if you choose downloading the zip file, the zip format will break all the soft link by changing them into plain text files.

How to fix it

In order to fix it, I wrote several lines of python. Here is the code:

#!/usr/bin/env python
# -*- encoding: utf-8 -*-
# vim: set et sw=4 ts=4 sts=4 ff=unix fenc=utf8:
# Author: Jeswang<wangyi724@gmail.com>
#         http://blog.jeswang.org
# Created on 2014-10-22 13:03:12

import os

def file_exists(filename):
        with open(filename) as f:
            return True
    except IOError:
        return False

def folder_exists(foldername):
        return True
    except StopIteration:
        return False

def generateSoftLink(realfile, linkfile):
    print("%s -> %s" % (linkfile, realfile))
    os.symlink(realfile, linkfile)

def generateSoftLinkForPath(path):
    list_of_files = {}
    for (dirpath, dirnames, filenames) in os.walk(path):
        for filename in filenames:
            print filename
            linkfile = dirpath+"/"+filename

            if os.path.islink(linkfile) or os.path.getsize(linkfile) > 200:

            f = open(linkfile)
            relpath = f.readline()

            if len(relpath) == 0:

            relpath = relpath.replace("\n", "")

            if len(relpath.replace("/", "")) == 0:

            realfile = dirpath+"/"+relpath

            if file_exists(realfile) or folder_exists(realfile):
                generateSoftLink(relpath, linkfile)

if __name__ == "__main__":

The tricky part is, under mac os x python’s function os.path.exists can’t deal well with soft links, so you have to test whether there is a file or folder in given path by trying to read the file or list the folder.

Because of the recursion of references, you may need to run this script several time to make sure all the soft link have be changed back.

- EOF -

(++a)+(++a)+(++a) 的计算结果


#include <iostream>
using namespace std;
int main(int argc, char *argv[]) {
    int a = 0;
    int b = (++a) + (++a) + (++a);
    return 0;

面试中被问到的 iOS 开发相关的问题

说来惭愧,已经面了几个公司的 iOS 开发岗位,几次被问倒——有些问题回答得比较含糊,有些问题只知道结果,不知道原理。根据回忆,梳理一下不太明白的一些问题,尽管对问题的解答只是点到为止,也算为以后的学习留点线索。

LeetCode for iOS 1.1 版


1.1.0 版发布说明:

  • 新功能:
    • 查看每道题给出的样例解法了
    • 支持 iPhone 6/6 Plus
  • 修正 Bug:
    • iPad 上的搜索动画修复




一共尝试了 3 种解法:

  • 使用两个 Map 记录是否访问过了左右子树(当然也可以放到一个 Map 中来)
  • 只用一个栈以及上次访问的节点做后序遍历
  • 传统的递归方法,用于验证