470. Implement Rand10() Using Rand7()

/**
 * The rand7() API is already defined in the parent class SolBase.
 * public int rand7();
 * @return a random integer in the range 1 to 7
 */
class Solution extends SolBase {
    public int rand10() {
        int index = Integer.MAX_VALUE;
        while (index >= 40) { // work for 0-39
            index = 7 * (rand7() - 1) + (rand7() - 1);
        }
        return index % 10 + 1;
    }
}