# Friends Of Appropriate Ages Problem

## Description

LeetCode Problem 825.

There are n persons on a social media website. You are given an integer array ages where ages[i] is the age of the i^th 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[x] + 7
• age[y] > age[x]
• age[y] > 100 && age[x] < 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:

 1 2 3 Input: ages = [16,16] Output: 2 Explanation: 2 people friend request each other.

Example 2:

 1 2 3 Input: ages = [16,17,18] Output: 2 Explanation: Friend requests are made 17 -> 16, 18 -> 17.

Example 3:

 1 2 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

## Sample C++ Code

 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 class Solution { public: int numFriendRequests(vector& ages) { unordered_map count; for (int &age : ages) count[age]++; int res = 0; for (auto &a : count) for (auto &b : count) if (request(a.first, b.first)) res += a.second * (b.second - (a.first == b.first ? 1 : 0)); return res; } bool request(int a, int b) { return !(b <= 0.5 * a + 7 || b > a || (b > 100 && a < 100)); } };