Implement Rand10() Using Rand7() — LeetCode 470 Python Solution

The problem

Given the API rand7() that generates a uniform random integer in the range [1, 7], write a function rand10() that generates a uniform random integer in the range [1, 10]. You can only call the API rand7(), and you shouldn't call any other API.

Example

Input

n = 1

Output

[2]

Python solution

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


class Solution:
    def rand10(self):
        """
        :rtype: int
        """
        while 1:
            i = rand7() - 1
            j = rand7()
            x = i * 7 + j
            if x <= 40:
                return x % 10 + 1

Complexity

Pattern: Math and Number Theory

Find the closed form, the invariant, or the modular identity — and skip the loop entirely. LeetCode 470. Implement Rand10() Using Rand7() is filed here on both counts: the reference solution below belongs to the algorithm family this hub collects, and LeetCode tags it Math.

The math and number theory guide has the Python template for the pattern and the 485 LeetCode problems that use it.

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