LeetCode practice: random

Leetcode practice of the random topic.

Leetcode 470. Implement Rand10() Using Rand7()

Given a function rand7 which generates a uniform random integer in the range 1 to 7, write a function rand10 which generates a uniform random integer in the range 1 to 10.
Do NOT use system’s Math.random().

# The rand7() API is already defined for you.
# def rand7():
# @return a random integer in the range 1 to 7

class Solution(object):
    def rand10(self):
        """
        :rtype: int
        """
        # the maximum common multiple n*10 (n=4) that is less than 49 (7*7)
        reject_threshold = 40
        # threshold <= res <= 7*7, to enter the following loop
        res = 41
        while res > reject_threshold:
            res = (rand7()-1)*7 + rand7() # generate 0 - 7*7
        return res%10 + 1