[Swift]LeetCode507. 完美数 | Perfect Number

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We define the Perfect Number is a positive integer that is equal to the sum of all its positive divisors except itself.

Now, given an integer n, write a function that returns true when it is a perfect number and false when it is not.

 Example:

Input: 28
Output: True
Explanation: 28 = 1 + 2 + 4 + 7 + 14

 Note: The input number n will not exceed 100,000,000. (1e8)

---

对于一个 正整数，如果它和除了它自身以外的所有正因子之和相等，我们称它为“完美数”。

给定一个 正整数 n， 如果他是完美数，返回 True，否则返回 False

 示例：

输入: 28
输出: True
解释: 28 = 1 + 2 + 4 + 7 + 14

 注意:

输入的数字 n 不会超过 100,000,000. (1e8)

---

 12ms

class Solution {
    func checkPerfectNumber(_ num: Int) -> Bool {
        if num < 2
        {
            return false
        }
        let n:Int = Int(sqrt(Double(num)))
        if n < 2
        {
            return false
        }
        var ans:Int = 1
        for i in 2...n
        {
            if num % i == 0
            {
                ans += i
                if num / i != i
                {
                    ans += num / i
                }
            }
        }
        return  ans == num
    }
}

---

12ms

class Solution {
    func checkPerfectNumber(_ num: Int) -> Bool {
        
        guard num > 1 else { return false }
        
        var sum = 0
        var i = 1
        while Double(i) <= sqrt(Double(num)) {
            let isDivisor = (num % i == 0)
            if isDivisor {
                let d1 = i
                let d2 = num / i
                sum = sum + d1
                if d2 != num && d2 != d1 {
                    sum = sum + d2
                }
                if sum > num { return false }
            }
            i = i + 1
        }
        let isPerfect = sum == num
        return isPerfect    
    }
}

---

16ms

class Solution {
    func checkPerfectNumber(_ num: Int) -> Bool {
        if num == 6 || num == 28 || num == 496 || num == 8128 || num == 33550336 {
            return true
        }
        return false
    }
}

---

20ms

class Solution {
    func checkPerfectNumber(_ num: Int) -> Bool {
        if num <= 1 {
            return false
        }
        
        let value = sqrt(Double(num))
        if value.isNaN || value.isInfinite {
            return false
        }
        
        let max = Int(value)
        if max < 2 {
            return false
        }
        
        var tempValue: Int = 1
        for index in 2 ... max {
            if num % index == 0 {
                tempValue += index
                tempValue += num / index
            }
        }
        
        return tempValue == num
    }
}

---

28ms

class Solution {
    func f(_ num: Int) -> Bool {
        if (num <= 1) {
            return false
        }
        var a = 0
        var max_num = Int(sqrt(Double(num)))
        a += 1
        if (2 <= max_num){
            for i in 2...max_num {
                if num % i == 0 {
                    a += i
                    if i != num / i {
                        a += num / i
                    }
                }
            }
        }
        
        return a == num
    }
    
    
    func checkPerfectNumber(_ num: Int) -> Bool {
        return f(num)
    }
}