825. Friends Of Appropriate Ages

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LeetCode

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Problem

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:

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 * 104
• 1 <= ages[i] <= 120

Code

class Solution {
    public int numFriendRequests(int[] ages) {
        Map<Integer, Integer> count = new HashMap<>();

        for (int age : ages) {
            count.put(age, count.getOrDefault(age, 0) + 1);
        }

        int res = 0;
        for (Integer x : count.keySet())
            for (Integer y : count.keySet())
                if (isSendRequest(x, y)) {
                    res += count.get(x) * count.get(y);
                    if (x == y) {
                        res -= count.get(x);
                    }
                }

        return res;
    }

    private boolean isSendRequest(int x, int y) {
        return !(y <= 0.5 * x + 7 || y > x || (y > 100 && x < 100));
    }
}