Common elements P12675

Write a function

    int common_elements(const vector<int>& X, const vector<int>& Y);

that given two vectors @X@ and @Y@ in increasing order, returns the number of common elements in the two vectors, that is, the number of integer numbers $a$ such that $a=X[i]=Y[j]$ for any $i$ and $j$.

Precondition

The two vectors are in strictly increasing order.

Samples

@common_elements@$([3,5,7,8], [2,3,7,9,10]) = 2.$
@common_elements@$([1,2,3,4,5], [3,4,5,6,7,8]) = 3.$
@common_elements@$([1,2,3,4,5], [8,9]) = 0.$