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

Example 2:

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

Example 3:

``````1
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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

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class Solution {
public:
int numFriendRequests(vector<int>& ages) {
unordered_map<int, int> 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));
}
};
``````