Friends Of Appropriate Ages - LeetCode solutions

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 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.

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

Solution (Java)

class Solution {
    public int numFriendRequests(int[] ages) {
        int[] counter = new int[121];
        for (int k : ages) {
            counter[k]++;
        }

        int result = 0;
        for (int k = 15; k < counter.length; k++) {
            if (counter[k] == 0) {
                continue;
            }
            result += counter[k] * (counter[k] - 1);

            for (int y = k - 1; y > k / 2.0 + 7; y--) {
                result += counter[k] * counter[y];
            }
        }
        return result;
    }
}

Explain:

nope.

Complexity:

Time complexity : O(n).
Space complexity : O(n).