Number Operation

Give an array with 1 to N except 1 number. Find the missing number.

public static int FindMissingNumber(int[] a)
{
    if (a == null || a.Length <= 1)
        return 0;

    int sum = 0;
    for (int i = 0; i < a.Length; i++)
        sum = sum ^ a[i] ^ (i+1);

    return sum ^ (a.Length+1);
}

This solution is better:

a ={1,3}   Missing 2

using XOR characteristic when XOR same= number it will become 0 :  A XOR A = 0.

1st pass : 1 XOR 1 = 0

001 XOR 001 = 000

2nd pass : 0 XOR 3 XOR 2 = 1        

000 XOR 011 XOR 010 = 001

Final : 1 XOR 3 = 2

001 XOR 011 = 010

* Avoid using n*(n+1)/2 – sum approach as this will introduce overflow for integer data type.