Minimum circular rotations to obtain a given numeric string by avoiding a set of given strings

// C# Program to count the minimum
// number of circular rotations required
// to obtain a given numeric strings
// avoiding a set of blocked strings
using System;
using System.Collections.Generic;
using System.Text;

public class Test
{
  // Function to reorder elements of arr[] according
  // to index[]
  static int minCircularRotations(string target, string[] blocked, int N)
  {
    string start = "";
    for (int i = 0; i < N; i++) {
      start += '0';
    }

    HashSet < string > avoid = new HashSet < string >();

    for (int i=0; i<blocked.Length; i++)
    {
      avoid.Add(blocked[i]);
      // If the starting string needs
      // to be avoided

      if (avoid.Contains(start))
        return -1;

      // If the final string needs
      // to be avoided
      if (avoid.Contains(target))
        return -1;
    }

    Queue<string> qu = new Queue<string>();
    qu.Enqueue(start);

    // Variable to store count of rotations
    int count = 0;

    // BFS Approach
    while (qu.Count != 0) {

      count++;

      // Store the current size
      // of the queue
      int size = qu.Count;

      for (int j = 0; j < size; j++) {

        string st = qu.Peek();
        StringBuilder sb = new StringBuilder(st);
        qu.Dequeue();

        // Traverse the string
        for (int i = 0; i < N; i++) {

          char ch = st[i];

          // Increase the
          // current character
          char c=ch;
          sb[i] = ++c;
          st = sb.ToString();

          // Circular rotation
          if (st[i] > '9')
          {
            sb[i] = '0';
            st = sb.ToString();
          }

          // If target is reached
          if (st == target)
            return count;

          // If the string formed
          // is not one to be avoided
          if (!avoid.Contains(st))
            qu.Enqueue(st);

          // Add it to the list of
          // strings to be avoided
          // to prevent visiting
          // already visited states
          avoid.Add(st);

          // Decrease the current
          // value by 1 and repeat
          // the similar checkings
          c=ch;
          sb[i] = --c;
          st = sb.ToString();

          if (st[i] < '0')
          {
            sb[i] = '9';
            st = sb.ToString();
          }
          if (st == target)
            return count;
          if (!avoid.Contains(st))
            qu.Enqueue(st);
          avoid.Add(st);

          // Restore the original
          // character
          sb[i] = ch;
          st = sb.ToString();
        }
      }
    }

    return -1;
  }

  // Driver Code
  static void Main()
  {
    int n=4;
    string target = "7531";
    string[] blocked = new string[]{ "1543",
                                    "7434",
                                    "7300",
                                    "7321",
                                    "2427" };


    Console.WriteLine(minCircularRotations(target,blocked, n));
  }
}

// This code is contributed by Aditya_Kumar