[LC] 825. Friends Of Appropriate Ages

Some people will make friend requests. The list of their ages is given and ages[i] is the age of the ith person. 

Person A will NOT friend request person B (B != A) if any of the following conditions are true:

• age[B] <= 0.5 * age[A] + 7
• age[B] > age[A]
• age[B] > 100 && age[A] < 100

Otherwise, A will friend request B.

Note that if A requests B, B does not necessarily request A.  Also, people will not friend request themselves.

How many total friend requests are made?

Example 1:

Input: [16,16]
Output: 2
Explanation: 2 people friend request each other.

Example 2:

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

Example 3:

Input: [20,30,100,110,120]
Output: 
Explanation: Friend requests are made 110 -> 100, 120 -> 110, 120 -> 100.

Notes:

• 1 <= ages.length <= 20000.
• 1 <= ages[i] <= 120.

class Solution {
    public int numFriendRequests(int[] ages) {
        Map<Integer, Integer> map = new HashMap<>();
        int ans = 0;
        for (int num : ages) {
            map.put(num, map.getOrDefault(num, 0) + 1);
        }
        for (int a: map.keySet()) {
            for (int b: map.keySet()) {
                if (!isValid(a, b)) {
                    continue;
                }
                if (a == b) {
                    ans += map.get(a) * (map.get(a) - 1);
                } else {
                    ans += map.get(a) * map.get(b);
                }
            }
        }
        return ans;
    }
    
    private boolean isValid(int a, int b) {
        if (b <= 0.5 * a + 7 || b > a || b > a && a < 100) {
            return false;
        }
        return true;
    }
}