[LeetCode] 825. Friends Of Appropriate Ages

Problem

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.

Solution

class Solution {
    public int numFriendRequests(int[] ages) {
        //0.5*ages[A]+7 < ages[B] <= ages[A]
        //7 < 0.5*ages[A] ====> ages[A] >= 15
        int res = 0;
        int[] nums = new int[121];
        int[] sums = new int[121];
        
        for (int age: ages) {
            nums[age]++;
        }
        
        for (int i = 1; i <= 120; i++) {
            sums[i] = sums[i-1]+nums[i];
        }
        
        for (int i = 15; i <= 120; i++) {
            if (nums[i] == 0) continue;
            int count = sums[i] - sums[i/2+7]; //number of range B
            res += count*nums[i] - nums[i]; //range B * number of age A
        }
        
        return res;
    }
}