useful algorithms

Finding the duplicate digits in a number.cs

using System;
					
public class Program
{
	public static void Main()
	{
		var n = 124123;
		var countArray = new int[10];

		do {
			// get the last digit in the number
  			var rightmostDigit = n % 10;
			Console.WriteLine("rightmostDigit:" + rightmostDigit);
  
  			countArray[rightmostDigit]++;
			// remove the last digit in the number
			n = n / 10;
			Console.WriteLine("n:" + n);
		} while(n != 0);

		// now that we have stored the occurrences
		// of each of the digits inside the `countArray`,
		// we just have to loop through the elements
		// of that array
		for(var i = 0; i < 10; i++) {
  
	  		// check if the value is greater than 1 (this
	  		// digit existed more than once, hence a duplicate)
	  		if(countArray[i] > 1) {
    
	    		// take note that the index represents the digit,
		    	// while the element represents how many times
		    	// that digit has occurred
		    	Console.WriteLine("Digit " + i + " existed " + countArray[i] + " times\n"); 
		  	}
		}
	}
}