Problem description:

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?

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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:

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

Solution:

Use a map to gather all the same ages together, then use a function to check whether if the input ages will send request.

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class Solution {
public:
    int numFriendRequests(vector<int>& ages) {
        unordered_map<int, int> count;
        for(auto &a: ages)
            count[a]++; //calculate the same ages together
        
        int res= 0;
        for(auto x: count){
            for(auto y: count){
                if(request(x.first, y.first))
                    res += x.second * (y.second - (x.first == y.first ? 1 : 0));
            }
        }
        
        return res;
    }
    
    bool request(int a, int b){
        return !(b <= 0.5 * a + 7 || b > a || (b > 100 && a < 100));
    }
};

time complexity: $O(n^2)$
space complexity: $O(n)$
reference:
https://goo.gl/XQgtCY