LeetCode 470. Implement Rand10() Using Rand7()

原题链接在这里：https://leetcode.com/problems/implement-rand10-using-rand7/

题目：

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()?

题解：

Having one rand7() - 1, a, generated (0 - 6) and second rand7() - 1, b, generated, 7 * a + b could be [0 - 48].

And drawing a 2D array, find out it is evenly distributed on each number of [0 - 48]. If it is >= 40, regenerate.

Otherwise, use it % 10 + 1 would be result, random between [1, 10].

Time Complexity: O(1). 

Space: O(1).

AC Java:

/**
 * The rand7() API is already defined in the parent class SolBase.
 * public int rand7();
 * @return a random integer in the range 1 to 7
 */
class Solution extends SolBase {
    public int rand10() {
        int ind = Integer.MAX_VALUE;
        while(ind >= 40){
            ind = 7 * (rand7() - 1) + (rand7() - 1);
        }
        
        return ind % 10 + 1;
    }
}