Implement Rand10() Using Rand7().cpp

// The rand7() API is already defined for you.
// int rand7();
// @return a random integer in the range 1 to 7

class Solution {
public:
    int rand10() {
        int num = 40;
        while (num >= 40) 
            num = 7 * (rand7() - 1) + (rand7() - 1);
        return num % 10 + 1;
    }
};