fail to install myql gem for ruby

You tried to install mysql gem on linux:

$ gem install mysql

Building native extensions.  This could take a while...
ERROR:  Error installing mysql:
	ERROR: Failed to build gem native extension.

/home/t/ruby-1.9.2-p136-1/bin/ruby extconf.rb
checking for mysql_ssl_set()... *** extconf.rb failed ***
Could not create Makefile due to some reason, probably lack of
necessary libraries and/or headers.  Check the mkmf.log file for more
details.  You may need configuration options.

Provided configuration options:
	--with-opt-dir
	--without-opt-dir
	--with-opt-include
	--without-opt-include=${opt-dir}/include
	--with-opt-lib
	--without-opt-lib=${opt-dir}/lib
	--with-make-prog
	--without-make-prog
	--srcdir=.
	--curdir
	--ruby=/home/t/ruby-1.9.2-p136-1/bin/ruby
	--with-mysql-config
	--without-mysql-config
/home/t/ruby-1.9.2-p136-1/lib/ruby/1.9.1/mkmf.rb:368:in `try_do': The complier failed to generate an executable file. (RuntimeError)
You have to install development tools first.
	from /home/t/ruby-1.9.2-p136-1/lib/ruby/1.9.1/mkmf.rb:435:in `try_link0'
	from /home/t/ruby-1.9.2-p136-1/lib/ruby/1.9.1/mkmf.rb:440:in `try_link'
	from /home/t/ruby-1.9.2-p136-1/lib/ruby/1.9.1/mkmf.rb:552:in `try_func'
	from /home/t/ruby-1.9.2-p136-1/lib/ruby/1.9.1/mkmf.rb:797:in `block in have_func'
	from /home/t/ruby-1.9.2-p136-1/lib/ruby/1.9.1/mkmf.rb:693:in `block in checking_for'
	from /home/t/ruby-1.9.2-p136-1/lib/ruby/1.9.1/mkmf.rb:280:in `block (2 levels) in postpone'
	from /home/t/ruby-1.9.2-p136-1/lib/ruby/1.9.1/mkmf.rb:254:in `open'
	from /home/t/ruby-1.9.2-p136-1/lib/ruby/1.9.1/mkmf.rb:280:in `block in postpone'
	from /home/t/ruby-1.9.2-p136-1/lib/ruby/1.9.1/mkmf.rb:254:in `open'
	from /home/t/ruby-1.9.2-p136-1/lib/ruby/1.9.1/mkmf.rb:276:in `postpone'
	from /home/t/ruby-1.9.2-p136-1/lib/ruby/1.9.1/mkmf.rb:692:in `checking_for'
	from /home/t/ruby-1.9.2-p136-1/lib/ruby/1.9.1/mkmf.rb:796:in `have_func'
	from extconf.rb:50:in `<main>'


Gem files will remain installed in /home/t/ruby-1.9.2-p136-1/lib/ruby/gems/1.9.1/gems/mysql-2.8.1 for inspection.
Results logged to /home/t/ruby-1.9.2-p136-1/lib/ruby/gems/1.9.1/gems/mysql-2.8.1/ext/mysql_api/gem_make.out
 

You need to check mkmf.log which is in the same folder as gem_make.out is in.  In my case, mkmf.log is in /home/t/ruby-1.9.2-p136-1/lib/ruby/gems/1.9.1/gems/mysql-2.8.1/ext/mysql_api/ 

Read mkmf.log, if you see something similar to below: 

/usr/bin/ld: skipping incompatible /usr/lib/libz.so when searching for -lz
/usr/bin/ld: skipping incompatible /usr/lib/libz.a when searching for -lz
/usr/bin/ld: cannot find -lz
collect2: ld returned 1 exit status

It means you have an incompatible zlib-devel binary.  In case, the 32-bit zlib-devel is installed, but not the 64 bits.  Run following command to fix it:

yum erase zlib-devel
yum install zlib-devel 

Reverse a singly linked list

I was asked this question during an interview today.  I didn't do well though.  I guess because it was an interview, I had some pressure and could think fast and clearly.  In fact, this question is pretty easy with the recursive.  When I got home, it didn't take me too long to figure out the correct answer.  Here is the code in Java.

class Node {
    Node next;
}

class Util {
    public static void reverseNode(Node node) {
        reverseNode(node, null);
    }
   
    private static void reverseNode(Node node, Node previous) {
        if (node != null) {
            reverseNode(node.next, node);
            node.next = previous;
        }
    }
}

Update:

The recursive solution is easy to understand and implement.  However, you may get a StackOverflow exception if the list is big.  Here is the non-recursive solution:

class Util {
    public static void reverseNode(Node node) {
        Node previous = null;
        while (node != null) {
            Node next = node.next;
            node.next = previous;
            previous = node;
            node = next;
        }
    }
}

Swap 2 integers without using a temp variable

There are 2 ways to do it. 

1. use sum of the 2 variables:

 void swap(int a, int b) {

       a = a + b;

       b = a - b;

       a = a - b

    }

     2. use bitwise operation (xor)

void swap(int a, int b) {

      a = a ^ b;

      b = a ^ b;

      a = a ^ b;

    }

Create Singleton in Java

A singleton is simply a class that is instantiated exactly once.  There are two approaches to create a singleton class.

1. Early initialization,
2. Lazy initialization.

 There are several ways to create singleton in each approach.

For early initialization:

1. Singleton with public final field

public class Singleton {

      public static final Singleton INSTANCE = new Singleton();

      private Singleton() {

         //do something

      }

      public void someMethod() {

         // do something

      }

    }

     2. Singleton with static factory

public class Singleton {

     private static final Singleton INSTANCE = new Singleton();

     

     private Singleton() {

         //do something

     }

     public static Singleton getInstance(){

       return INSTANCE

     }

     public void someMethod() {

         // do something

     }   
   }
  

      3. Singleton with enum

public enum Singleton {

      INSTANCE;

            

      private Singleton() {

         //do something

      }

      public void someMethod() {

         // do something

      }    
    }


For lazy initialization:

  However unless you absolutely need it, don't use lazy initialization.

     1. Lazy initialization holder class idiom for static fields (a class will not be initialized until it is used)

public class LazySingleton {

     private static class SingletonHolder {

       static final LazySingleton INSTANCE = new LazySingleton();

     }

     

     private LazySingleton() {

         //do something

     }

     public static LazySingleton getInstance(){

       return SingletonHolder.INSTANCE;

     }

     public void someMethod() {

         // do something

     }
    }

 2. Double-check idiom for lazy initialization

   class LazySingleton {
     private static volatile LazySingleton instance;
     private LazySingleton(){
        //do something
     }

     public static LazySingleton getInstance() {
        if (instance == null) {
            synchronized (LazySingleton.class) {
                if (instance == null) {
                    INSTANCE = new LazySingleton();
                }
            }
        }
        return instance;

     }
   }

Reverse the string word by word, in place

The idea is reverse the string character by character first, then reverse the word character by character.  For example, we have string "abc defg hijk".  The first step is to reverse the string character by character:

"abc defg hijk" ==> "kjih gfed cba"

Then reverse the word in the output string character by character:

"kjih gfed cba" ==> "hijk defg abc"

The run time is O(n) with constant extra space.  Here is the code in java:

    public static void resverseString(char[] input) {
        reverseString(input, 0, input.length - 1);
        int start = 0;
        int end = 0;
        for (char temp : input) {
            if (temp == ' ') {
                reverseString(input, start, end - 1);
                start = end + 1;
            }
            end++;
        }
        reverseString(input, start, end - 1);
    }
   
    private static void reverseString(char[] input, int start, int end) {
        while (start < end) {
            swap(input, start, end);
            start++;
            end--;
        }
    }
   
    private static void swap (char[] input, int i, int j) {
        char temp = input[i];
        input[i] = input[j];
        input[j] = temp;
    }

Weak Reference in Java

Today, during a phone interview, I was asked a question about WeakReference in Java.  I only knew that the weak referenced object can be garbage collected by JVM, and nothing more.  I did a little research, and realized that there are 4 types of references in Java:

1. Strong Reference
2. Soft Reference
3. Weak Reference
4. Phantom Reference

They are in the order from strong to weak references.  3 classes, SoftReference, WeakReference, and PhantomReference are for the last 3 references.  For strong reference, just use new operator. Below is a great blog talking about all 4 references:

Understanding Weak References

Print a sequence of Fibonacci number

The trick is that you should use BigInteger instead of long.  Otherwise, you will get overflow pretty soon.  Here is the code:

import java.math.BigInteger;

public class Fibonacci {
    /**
     * print out a list of Fibonacci sequence to n places.
     * if n = 0, no number will be printed out.  if n = 1, print the first Fibonacci number.
     *
     * @param n the number of Fibonacci to be printed out
     */
    public static void print(int n) {
        BigInteger f0 = BigInteger.ZERO;
        BigInteger f1 = BigInteger.ONE;
        for (int i = 0; i < n; i++) {
           
            System.out.print(f0);
            if (i != n - 1) {
                System.out.print(", ");
            }
            BigInteger temp = f1;
            f1 = f1.add(f0);
            f0 = temp;
        }
    }
}

Update: Another thing to consider is that don't use recursive method to compute Fibonacci number because it is too expensive.  The run time is 2^n to recursively compute Fibancci number.