[Swift]LeetCode825. 适龄的朋友 | Friends Of Appropriate Ages

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Some people will make friend requests. The list of their ages is given and ages[i] is the age of the ith person. 

Person A will NOT friend request person B (B != A) if any of the following conditions are true:

• age[B] <= 0.5 * age[A] + 7
• age[B] > age[A]
• age[B] > 100 && age[A] < 100

Otherwise, A will friend request B.

Note that if A requests B, B does not necessarily request A.  Also, people will not friend request themselves.

How many total friend requests are made?

Example 1:

Input: [16,16]
Output: 2
Explanation: 2 people friend request each other.

Example 2:

Input: [16,17,18]
Output: 2
Explanation: Friend requests are made 17 -> 16, 18 -> 17.

Example 3:

Input: [20,30,100,110,120]
Output: 
Explanation: Friend requests are made 110 -> 100, 120 -> 110, 120 -> 100. 

Notes:

• 1 <= ages.length <= 20000.
• 1 <= ages[i] <= 120.

---

人们会互相发送好友请求，现在给定一个包含有他们年龄的数组，ages[i] 表示第 i 个人的年龄。

当满足以下条件时，A 不能给 B（A、B不为同一人）发送好友请求：

• age[B] <= 0.5 * age[A] + 7
• age[B] > age[A]
• age[B] > 100 && age[A] < 100

否则，A 可以给 B 发送好友请求。

注意如果 A 向 B 发出了请求，不等于 B 接受了 A 的请求。而且，人们不会给自己发送好友请求。 

求总共会发出多少份好友请求?

示例 1:

输入: [16,16]
输出: 2
解释: 二人可以互发好友申请。

示例 2:

输入: [16,17,18]
输出: 2
解释: 好友请求可产生于 17 -> 16, 18 -> 17.

示例 3:

输入: [20,30,100,110,120]
输出: 3
解释: 好友请求可产生于 110 -> 100, 120 -> 110, 120 -> 100.

说明:

• 1 <= ages.length <= 20000.
• 1 <= ages[i] <= 120.

---

Runtime: 240 ms

Memory Usage: 19.4 MB

class Solution {
    func numFriendRequests(_ ages: [Int]) -> Int {
        var res:Int = 0
        var numInAge:[Int] = [Int](repeating:0,count:121)
        var sumInAge:[Int] = [Int](repeating:0,count:121)
        for age in ages
        {
            numInAge[age] += 1
        }
        for i in 1...120
        {
            sumInAge[i] = numInAge[i] + sumInAge[i - 1]
        }
        for i in 15...120
        {
            if numInAge[i] == 0 {continue}
            var cnt:Int = sumInAge[i] - sumInAge[Int(Double(i) * 0.5 + 7)]
            res += cnt * numInAge[i] - numInAge[i]
        }
        return res
    }
}

---

268ms

class Solution {
    func numFriendRequests(_ ages: [Int]) -> Int {
        var count = [Int](repeating: 0, count: 121)
        for age in ages { count[age] += 1 }

        var ans = 0
        for ageA in 0...120 {
            let countA = count[ageA]
            for ageB in 0...120 {
                let countB = count[ageB]
                if (ageA + 14) >= 2*ageB { continue }
                if ageA < ageB { continue }
                if ageA < 100 && 100 < ageB { continue }
                ans += countA * countB
                if ageA == ageB  { ans -= countA}
            }
        }
        return ans
    }
}

---

292ms

class Solution {
    func numFriendRequests(_ ages: [Int]) -> Int {
        var map = [Int: Int]()
        for age in ages {
            map[age] = (map[age] ?? 0) + 1
        }
        var result = 0
        for i in 0...120 {
            if let iCount = map[i] {
                for j in 0...120 {
                    if let jCount = map[j] {
                        if j <= i / 2 + 7 || j > i || (j > 100 && i < 100) {
                            continue
                        }
                        if i == j {
                            result += (iCount - 1) * iCount
                        } else {
                            result += iCount * jCount
                        }
                    }
                }
            }
        }
        return result
    }
}

---

404ms

class Solution {
    func numFriendRequests(_ ages: [Int]) -> Int {
        var ageCounts = [Int](repeating: 0, count: 121)
        for age in ages {
            ageCounts[age] += 1
        }
        var res = 0
        for i in 0..<ageCounts.count {
            var ageA = i
            for j in 0..<ageCounts.count {
                var ageB = j
                if ageB <= Int(floor(0.5 * Double(ageA) + 7)) { continue }
                if ageB > ageA { continue }
                res += ageCounts[i] * ageCounts[j]
                if ageA == ageB {
                    res -= ageCounts[i]
                }
            }
        }
        return res
    }
}