C# Program - Find missing number in Array

Missing Number If the array contains numbers from 1 to N with one missing element, you can solve it in multiple ways.

Example

  int[] a = { 1, 2, 4, 5 };
  

Output

  Missing Number = 3
  

Approach 1: Sum Formula (Best - O(n))

using System;

class Program
{
    static void Main()
    {
        int[] a = { 1, 2, 4, 5 };

        int n = a.Length + 1;

        int expectedSum = n * (n + 1) / 2;

        int actualSum = 0;

        foreach (int num in a)
        {
            actualSum += num;
        }

        Console.WriteLine("Missing Number = " + (expectedSum - actualSum));
    }
}
  

Output

  Missing Number = 3
  

Complexity

1. Time: O(n)
2. Space: O(1)

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