[C#] Download files from website.

We have two ways to download files from website by using .net framework.

1. WebClient.

using System.Net;

private void button1_Click(object sender, EventArgs e)
{
    WebClient wc = new WebClient();
    wc.DownloadFile("http://www.taifex.com.tw/DailyDownload/Daily_2013_05_24.zip", "d:\\Daily_2013_05_24.zip");
}

2. HttpRequest + Stream

using System.IO;
using System.Net;

private void button1_Click(object sender, EventArgs e)
{
    string url = "http://www.taifex.com.tw/DailyDownload/Daily_2013_05_24.zip";
    HttpWebRequest httpRequest = (HttpWebRequest)WebRequest.Create(url);
    HttpWebResponse httpResponse = (HttpWebResponse)httpRequest.GetResponse();

    System.IO.Stream dataStream = httpResponse.GetResponseStream();
    byte[] buffer = new byte[8192];

    FileStream fs = new FileStream("d:\\Daily_2013_05_24.zip", FileMode.Create, FileAccess.Write);
    int size = 0;
    do
    {
        size = dataStream.Read(buffer, 0, buffer.Length);
        if (size > 0)
            fs.Write(buffer, 0, size);
    } while (size > 0);
    fs.Close();

    httpResponse.Close();

    Console.WriteLine("Done at " + DateTime.Now.ToString("HH:mm:ss.fff"));
}

Reference:
http://blog.darkthread.net/post-2008-10-14-download-file-with-c.aspx

N Coin Problem

Question:
Given a list of 'N' coins, their values being in an array A[], return the minimum number of coins required to sum to 'S' (you can use as many coins you want). If it's not possible to sum to 'S', return -1

For Example, input N coins array { 1, 3, 5 } and S as 11, the answer should be 3


My Answer:  (I am not sure if it is correct.)

int minCoins(int* a, int count, int target)
{
    int N = count;
    int S = target;

    int *mina=NULL;
    mina = new int[S+1];
 
    mina[0]=0;
     
    for(int i=1;i<=S;i++)
    {
        mina[i]=-1;
 
        for(int j=0;j<N;j++)
        {
            if(a[j]<=i && mina[i-a[j]] != -1)  
            {
                if(mina[i]==-1 || mina[i-a[j]]+1 < mina[i])
                {
                   mina[i] = mina[i-a[j]]+1;
                }
            }
        }
    }
 
    return mina[S];
}

Circle sorted array searching.

Question:
Given a circle sorted array, please write a function to search a number and output its position.

Example:
Find number 6 in array { 1,2,3,4,5,6,7 }, output is 5
Find number 6 in array { 5,6,7,1,2,3,4 }, output is 1

My Answer:  (I am no sure if it is correct.)

#include "stdafx.h"
#include 

using namespace std;

int binarySearch(int n, int* a, int l, int r);

int _tmain(int argc, _TCHAR* argv[])
{
 int a[7] = { 4, 5, 6, 7, 1, 2, 3 };
 
 cout << binarySearch(6, a, 0, 6) << endl;
 cout << binarySearch(2, a, 0, 6) << endl;
 cout << binarySearch(5, a, 0, 6) << endl;

 cin.get();
 return 0;
}

int binarySearch(int n, int* a, int l, int r)
{
 int i = (l + r) / 2;
 if (n == a[i])
  return i;

 if (a[l] < a[r])
 {
  if (n > a[i])
  {
   l += 1;
  }
  else
  {
   r = i - 1;
  }
 }
 else
 {
  if (n > a[i] || n < a[l])
  {
   l += 1;
  }
  else
  {
   r = i - 1;
  }
 }

 return binarySearch(n, a, l, r);
}

Number Complement.

Question: 
A complement of a number is defined as inversion (if the bit value = 0, change it to 1 and vice-versa) of all bits of the number starting from the leftmost bit that is set to 1. 

For example, if N = 5, N is 101 in binary. The complement of N is 010, which is 2 in decimal. Similarly if N = 50, then complement of N is 13 
Complete the function getIntegerComplement(). This function takes N as it's parameter. The function should return the complement of N.  (The N >=0)


My Answer:  (I am not sure if it is correct.)

int getComplement(int n)
{
    if (n == 0)
        return 1;

    int b = 0;
    int a = n;
    while (a > 0)
    {
        a >>= 1;
        b++;
    };

    int mask = pow(2.0, b) - 1;
    int result = n ^ mask;
    return result;
}

Get Nth power of number.

Question:
Given two integer a and b (b >= 0), please write a function to return the result of "a to the power of b".

My Anwser: (I am no sure if it is correct.)
The easiest way is recursively multiply integer a.

int power(int a, int b)
{
    if (b <= 0)
        return 1;

    return a * power(a, b-1);
}

Question:
Improve time complexity to log(n)

My Anwser: (I am no sure if it is correct.)
Consider a to the power of 15 is power(a, 8) * power(a, 4) * power(a, 2) * power(a, 1) * power(a, 0)

#include "stdafx.h"
#include 

using namespace std;

int getPower(int a, int b);
int power(int a, int b);

int _tmain(int argc, _TCHAR* argv[])
{
    int a = 2, b = 15;
 
    cout << power(a, b) << endl;

    cin.get();
    return 0;
}

int getPower(int a, int logb)
{
    if (logb <= 0)
        return 1;

    return a * getPower(a*a, logb-1);
}

int power(int a, int b)
{
    if (b == 0)
        return 1;

    int logb = 0;
    while(b>0)
    {
        logb++;
        b >>= 1;
    }

    return getPower(a, logb);
}