825. Friends Of Appropriate Ages

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[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.

class Solution {
public:
    int numFriendRequests(vector<int>& A) {
        int res = 0;
        int freq[121] = {};
        for(auto& a : A) freq[a]++;
        for(int i = 15; i <= 120; i++) {
            for(int j = i * 0.5 + 8; j <= i; j++)
                res += freq[j] * (freq[i] - (i==j));
        }
        return res;
    }
};