Utility code for Hackerrank exercises

hackerrank.cs

// char array containing lower-case letters from a..z
var letters = new char[] {'a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z'};

// int to binary string with leading zeros
Convert.ToString(3, 2).PadLeft(4, '0'); // 0011
Convert.ToString(3, 2).PadLeft(8, '0'); // 00000011

// binary to int    
int output = Convert.ToInt32(input, 2);

// QuickSelect
static int QuickSelect(int[] arr, int left, int right, int k) 
{
    // array contains only one element
    if(left == right) 
    {
        return arr[left];
    }
    // select pivotIndex between left..right
    var pivotIndex = (right - left) / 2 + left;

    pivotIndex = Partition(arr, left, right, pivotIndex);

    // k is in its final sorted position
    if(pivotIndex == k) 
    {
        return arr[k];
    } else if(k < pivotIndex) { // check the left-side of pivot
        return QuickSelect(arr, left, pivotIndex - 1, k);
    } else { // check the right-side of pivot
        return QuickSelect(arr, pivotIndex + 1, right, k);
    }                
}

// partition
static int Partition(int[] arr, int left, int right, int pivotIndex) 
{
    var pivotValue = arr[pivotIndex];
    // move pivot to end
    Swap(ref arr[pivotIndex], ref arr[right]);
    var storeIndex = left;
    for(int i = left; i <= right - 1; i++) 
    {
        if(arr[i] < pivotValue)
        {
            Swap(ref arr[storeIndex], ref arr[i]);
            storeIndex++;
        }
    }
    // move pivot from end to it's final place
    Swap(ref arr[right], ref arr[storeIndex]);

    return storeIndex;
}