825. Friends Of Appropriate Ages

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825. Friends Of Appropriate Ages

题目描述

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:

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

Example 2:

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

Example 3:

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

题目大意

A 可以向 B请求是否可以做朋友，满足以下条件的一项，便不可做朋友：

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

Notes:

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

解题思路

记录每个年龄的人数，记为 ageCount
记录小于等于当前年龄的人数，记为 sumCount
由于题目要求 age[B] <= 0.5 * age[A] + 7，所以需要减去 <= 0.5 * age[A] + 7 的人数。
并且需要减去自身的个数。

代码

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class Solution {
    func numFriendRequests(_ ages: [Int]) -> Int {
        var ageCount = Array(repeating: 0, count: 121)
        var sumCount = Array(repeating: 0, count: 121)
        for age in ages { ageCount[age] += 1 }
        for i in 1...120 {
            sumCount[i] = sumCount[i - 1] + ageCount[i]
        }
        var ans = 0
        for i in 15...120 {
            if ageCount[i] == 0 { continue }
            let count = sumCount[i] - sumCount[i / 2 + 7]
            ans += count * ageCount[i] - ageCount[i]
        }
        return ans
    }
}
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