[Swift]LeetCode470. 用 Rand7() 实现 Rand10() | Implement Rand10() Using Rand7()

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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 rand10is 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()?

---

已有方法 rand7 可生成 1 到 7 范围内的均匀随机整数，试写一个方法 rand10 生成 1 到 10 范围内的均匀随机整数。

不要使用系统的 Math.random() 方法。 

示例 1:

输入: 1
输出: [7]

示例 2:

输入: 2
输出: [8,4]

示例 3:

输入: 3
输出: [8,1,10] 

提示:

1. rand7 已定义。
2. 传入参数: n 表示 rand10 的调用次数。 

进阶:

1. rand7()调用次数的 期望值 是多少 ?
2. 你能否尽量少调用 rand7() ?

---

Runtime: 44 ms

Memory Usage: 5.9 MB

/**
 * The rand7() API is already defined in the parent class SolBase.
 * func rand7() -> Int = {}
 * @return a random integer in the range 1 to 7
 */
class Solution : SolBase {
    func rand10() -> Int {
        while(true)
        {
            var a:Int = rand7()
            var b:Int = rand7()
            var num:Int = (a - 1) * 7 + b
            if num <= 40 {return num % 10 + 1}
            a = num - 40
            b = rand7()
            num = (a - 1) * 7 + b
            if num <= 60 {return num % 10 + 1}
            a = num - 60
            b = rand7()
            num = (a - 1) * 7 + b
            if num <= 20 {return num % 10 + 1}
        }
    }
}

---

44ms

/**
 * The rand7() API is already defined in the parent class SolBase.
 * func rand7() -> Int = {}
 * @return a random integer in the range 1 to 7
 */
class Solution : SolBase {
    func rand10() -> Int {
        var a, b, no: Int?
        repeat {
            a = rand7() - 1
            b = rand7() - 1
            no = 7 * a! + b!
        } while (no! > 39)
        
        return (no! % 10) + 1
    }
}

---

 48 ms

/**
 * The rand7() API is already defined in the parent class SolBase.
 * func rand7() -> Int = {}
 * @return a random integer in the range 1 to 7
 */
class Solution : SolBase {
    func rand10() -> Int {
        var num:Int = (rand7() - 1) * 7 + rand7()
        return (num <= 40) ? (num % 10 + 1) : rand10()        
    }
}