Friends Of Appropriate Ages.py

'''
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(object):
    def numFriendRequests(self, ages):
        """
        :type ages: List[int]
        :rtype: int
        """
        age_count = {}
        for age in ages:
            if age in age_count:
                age_count[age] += 1
            else:
                age_count[age] = 1
                
        res = 0
        for a in age_count:
            for b in xrange(int(0.5 * a + 7 + 1), a + 1):
                if b in age_count:
                    if a == b:
                        res += age_count[a] * (age_count[b] - 1)
                    else:
                        res += age_count[a] * age_count[b]
        
        return res