README.md

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().

Example 1:

Input: 1
Output: [7]

Example 2:

Input: 2
Output: [8,4]

Example 3:

Input: 3
Output: [8,1,10]

Note:

1. rand7 is predefined.
2. Each testcase has one argument: n, the number of times that rand10 is called.

Follow up:

1. What is the expected value for the number of calls to rand7() function?
2. Could you minimize the number of calls to rand7()?

Solutions (Rust)

1. Solution

/** 
 * The rand7() API is already defined for you.
 * @return a random integer in the range 1 to 7
 * fn rand7() -> i32;
 */

impl Solution {
    pub fn rand10() -> i32 {
        let mut a = rand7();
        let mut b = rand7() * 7;

        while a + b > 47 {
            a = rand7();
            b = rand7() * 7;
        }

        (a + b + 2) % 10 + 1
    }
}