動機

這三小

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. Please do not use a language's built-in random API.

Each test case will have one internal argument n, the number of times that your implemented function rand10() will be called while testing. Note that this is not an argument passed to rand10().

Follow up:

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

 

Example 1:

Input: n = 1Output: [2]

Example 2:

Input: n = 2Output: [2,8]

Example 3:

Input: n = 3Output: [3,8,10]

 

Constraints:

  • 1 <= n <= 105

Sol

為了讓機率平均，所以把1~49的組合對到1~10

但後面9個會多，所以要把後面9個擋掉

class Solution:
    def rand10(self):
        def get():
            i,j = rand7(), rand7()
            cnt = (i-1)*7+j
            ret = cnt%10
            return [ret if ret != 0 else 10, cnt < 41]
        
        ret, good = get()
        while not good:
            ret, good = get()
        return ret

另一個是看奇偶

int rand10(void)
{
	int a, b;

	do a = rand7(); while (a == 7);
	do b = rand7(); while (b > 5);

	return a & 1 ? b : b + 5;
}