470. Implement Rand10() Using Rand7()

# Time:  O(1.189), counted by statistics, limit would be O(log10/log7) = O(1.183)
# Space: O(1)

import random


def rand7():
    return random.randint(1, 7)


# Reference: https://leetcode.com/problems/implement-rand10-using-rand7/discuss/151567/C++JavaPython-Average-1.199-Call-rand7-Per-rand10
class Solution(object):
    def __init__(self):
        self.__cache = []

    def rand10(self):
        """
        :rtype: int
        """
        def generate(cache):
            n = 32
            curr = sum((rand7()-1) * (7**i) for i in xrange(n))
            rang = 7**n
            while curr < rang//10*10:
                cache.append(curr%10+1)
                curr /= 10
                rang /= 10

        while not self.__cache:
            generate(self.__cache)
        return self.__cache.pop()


# Time:  O(2 * (1 + (9/49) + (9/49)^2 + ...)) = O(2/(1-(9/49)) = O(2.45)
# Space: O(1)
class Solution2(object):
    def rand10(self):
        """
        :rtype: int
        """
        while True:
            x = (rand7()-1)*7 + (rand7()-1)
            if x < 40:
                return x%10 + 1

Solution from kamyu104/LeetCode-Solutions · MIT