825. Friends Of Appropriate Ages

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.

--------------------------
按年龄大小排序，O(n) time, O(1) space. 因为年龄是有限的1-120
题目中第3个条件是多余的

class Solution {
    public int numFriendRequests(int[] ages) {
        int[] ageMap = new int[121];
        
        for (int age : ages) {
            ageMap[age]++;
        }
        
        int rt = 0;
        
        for (int i = 120; i > 0; i--) {
            if (ageMap[i] == 0) continue;
            int cutOff = (int)(0.5 * i + 7) + 1;
            if (cutOff > i) continue;
            
            int cutOffNum = 0;
            for (int j = cutOff; j < i; j++) {
                cutOffNum += ageMap[j];
            }
            
            rt += cutOffNum * ageMap[i] + (ageMap[i] - 1) * ageMap[i];
        }
        
        return rt;
    }
}