Find Missing Number

Complexity o(n)
Input : arr[] = {2,4,1,5,6};
Output : 3
Code :
int arr[]={2,4,1,5,6};
int findMissingNumber(){
    int arrSize = sizeof(arr)/sizeof(int);
    int num = arr[0];
    int num2 = 1;
    int i;
    for( i= 1;i
            num ^=arr[i]; // xor all element of array
            num2 ^= (i+1); // xor all indexes of array from 2 to n+1
        }
    num2 ^= (i+1);
    int res = num^num2; // xor two results will give us the missing number from array
    return res;
    }