Compute number of distinct values in an array.

given an array $A$ consisting of $N$ integers, returns the number of distinct values in array A.

For example, given array $A$ consisting of six elements such that:

A[0] = 2 
A[1] = 1 
A[2] = 1 
A[3] = 2 
A[4] = 3 
A[5] = 1

the function should return $3$, because there are $3$ distinct values appearing in array A, namely $1, 2$ and $3$.

Write an efficient algorithm for the following assumptions:

• $N$ is an integer within the range $[0..100,000]$;
• each element of array $A$ is an integer within the range $[−1,000,000..1,000,000]$.