Finding first non-repeating number in integer array

Question

I got this question for an exam:

Given an integer array find the first number which is not repeating in array using O(N) time complexity and O(1) space co

Answer 1

If there are exactly TWO (or in multiples of 2) entries for all elements except one element, which will be non-repeating, you can use XOR operator.

Example:

int x=arr[0];
for(i=1;i<1000;i++)
  x^=a[i];
printf("Non-repeating: %d",x);

Any number XORed with itself is 0. So, if any number appears twice it will be 0 in the overall XOR result, thus leaving only the non-repeating number in x.

Note: If you have 1 million numbers, the variable to store the XOR result must be large enough.