LeetCode-Go/leetcode/0825.Friends-Of-Appropriate-Ages

825. Friends Of Appropriate Ages

题目

There are n persons on a social media website. You are given an integer array ages where ages[i] is the age of the ith person.

A Person x will not send a friend request to a person y (x != y) if any of the following conditions is true:

• age[y] <= 0.5 * age+ 7
• age[y] > age[x]
• age[y] > 100 && age< 100

Otherwise, x will send a friend request to y.

Note that if x sends a request to y, y will not necessarily send a request to x. Also, a person will not send a friend request to themself.

Return the total number of friend requests made.

Example 1:

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

Example 2:

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

Example 3:

Input: ages = [20,30,100,110,120]
Output: 3
Explanation: Friend requests are made 110 -> 100, 120 -> 110, 120 -> 100.

Constraints:

• n == ages.length
• 1 <= n <= 2 * 10^4
• 1 <= ages[i] <= 120

题目大意

在社交媒体网站上有 n 个用户。给你一个整数数组 ages ，其中 ages[i] 是第 i 个用户的年龄。

如果下述任意一个条件为真，那么用户 x 将不会向用户 y（x != y）发送好友请求：

• ages[y] <= 0.5 * ages[x] + 7
• ages[y] > ages[x]
• ages[y] > 100 && ages[x] < 100

否则，x 将会向 y 发送一条好友请求。注意，如果 x 向 y 发送一条好友请求，y 不必也向 x 发送一条好友请求。另外，用户不会向自己发送好友请求。返回在该社交媒体网站上产生的好友请求总数。

解题思路

• 简单题。先统计 [1,120] 范围内每个年龄的人数。然后利用题目中的三个判断条件，筛选符合条件的用户对。需要注意的是，相同年龄的人可以相互发送好友请求。不同年龄的人发送好友请求是单向的，即年龄老的向年龄轻的发送好友请求，年龄轻的不会对年龄老的发送好友请求。

代码

package leetcocde

func numFriendRequests(ages []int) int {
	res, count := 0, [125]int{}
	for _, x := range ages {
		count[x]++
	}
	for i := 1; i <= 120; i++ {
		for j := 1; j <= 120; j++ {
			if j > i {
				continue
			}
			if (j-7)*2 <= i {
				continue
			}
			if j > 100 && i < 100 {
				continue
			}
			if i != j {
				res += count[i] * count[j]
			} else {
				res += count[i] * (count[j] - 1)
			}
		}
	}
	return res
}