single number inside an array

singleNumber.cpp

/*Given an array of integers, every element appears twice except for one. Find that single one.

Note:
Your algorithm should have a linear runtime complexity. Could you implement it without using extra memory? 
*/

    int singleNumber(int A[], int n) {
        if (n <= 0)
            return -1;
        int a0 = A[0];
        for(int i=1; i<n; i++)
            a0 ^= A[i];
        
        return a0;
    }
    
    

Have to first check n <=0, otherwise a0=A[0] may crash for illegal memory access