题目：

题解：

class Solution:
    def rand10(self) -> int:
        while True:
            a = rand7()
            b = rand7()
            idx = (a - 1) * 7 + b
            if idx <= 40:
                return 1 + (idx - 1) % 10
            a = idx - 40
            b = rand7()
            # get uniform dist from 1 - 63
            idx = (a - 1) * 7 + b
            if idx <= 60:
                return 1 + (idx - 1) % 10
            a = idx - 60
            b = rand7()
            # get uniform dist from 1 - 21
            idx = (a - 1) * 7 + b
            if idx <= 20:
                return 1 + (idx - 1) % 10