Number which occurs only once in the array [duplicate]

Answer 1

Let the number which occurs only once in the array be x

x <- a[1]
for i <- 2 to n
   x <- x ^ a[i]
return x

Since a ^ a = 0 and a ^ 0 = a

Numbers which occur in pair cancel out and the result gets stored in x

Working code in C++

#include 

template
size_t size(T(&a)[N])
{
    return N;
}
int main()
{
   int a [] = {1,2,3,4,3,1,2};
   int x = a[0];
   for (size_t i = 1; i< size(a) ; ++i)
   {
      x = x ^ a[i];
   }
   std::cout << x;
}