Find missing number in a sequence in C#

Finding missing numbers in a sequence is a common programming problem. In C#, you can solve this efficiently using LINQ operations by comparing the original sequence with a complete range of numbers.

Using LINQ Except Method

The most straightforward approach is to create a complete range from the minimum to maximum value and use the Except method to find missing numbers −

using System;
using System.Collections.Generic;
using System.Linq;

public class Program {
   public static void Main() {
      List<int> myList = new List<int>(){1, 2, 3, 5, 8, 9};
      
      Console.WriteLine("Original Numbers:");
      foreach(int val in myList) {
         Console.WriteLine(val);
      }
      
      // Get the first and last elements
      int first = myList.Min();
      int last = myList.Max();
      
      // Create complete range and find missing numbers
      List<int> completeRange = Enumerable.Range(first, last - first + 1).ToList();
      List<int> missingNumbers = completeRange.Except(myList).ToList();
      
      Console.WriteLine("Missing Numbers:");
      foreach (int missing in missingNumbers) {
         Console.WriteLine(missing);
      }
   }
}

The output of the above code is −

Original Numbers:
1
2
3
5
8
9
Missing Numbers:
4
6
7

Using HashSet for Better Performance

For larger sequences, using a HashSet provides better performance with O(1) lookup time −

using System;
using System.Collections.Generic;
using System.Linq;

public class Program {
   public static void Main() {
      List<int> numbers = new List<int>(){10, 12, 14, 17, 20};
      
      HashSet<int> numberSet = new HashSet<int>(numbers);
      int min = numbers.Min();
      int max = numbers.Max();
      
      Console.WriteLine("Original sequence: " + string.Join(", ", numbers));
      Console.Write("Missing numbers: ");
      
      List<int> missing = new List<int>();
      for (int i = min; i <= max; i++) {
         if (!numberSet.Contains(i)) {
            missing.Add(i);
         }
      }
      
      Console.WriteLine(string.Join(", ", missing));
   }
}

The output of the above code is −

Original sequence: 10, 12, 14, 17, 20
Missing numbers: 11, 13, 15, 16, 18, 19

Finding Missing Number in Consecutive Sequence

For finding a single missing number in a consecutive sequence starting from 1, you can use the mathematical approach −

using System;
using System.Collections.Generic;
using System.Linq;

public class Program {
   public static void Main() {
      List<int> sequence = new List<int>(){1, 2, 3, 5, 6, 7, 8};
      
      int n = sequence.Count + 1; // Expected length
      int expectedSum = n * (n + 1) / 2;
      int actualSum = sequence.Sum();
      int missingNumber = expectedSum - actualSum;
      
      Console.WriteLine("Sequence: " + string.Join(", ", sequence));
      Console.WriteLine("Missing number: " + missingNumber);
   }
}

The output of the above code is −

Sequence: 1, 2, 3, 5, 6, 7, 8
Missing number: 4

Comparison of Methods

Conclusion

Finding missing numbers in a sequence can be achieved through various approaches in C#. Use LINQ's Except method for simplicity, HashSet for better performance with large datasets, or mathematical calculations for single missing numbers in consecutive sequences.