# 825. Friends Of Appropriate Ages 适龄的朋友

## # 题目描述：

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. 1 <= ages.length <= 20000.
2. 1 <= ages[i] <= 120.

## # 题目大意

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

## # 解题方法

``````class Solution(object):
def numFriendRequests(self, ages):
"""
:type ages: List[int]
:rtype: int
"""
count = collections.Counter(ages)
ages = sorted(count.keys())
N = len(ages)
res = 0
for A in ages:
for B in range(int(0.5 * A) + 7 + 1, A + 1):
res += count[A] * (count[B] - int(A == B))
return res
``````

## # 日期

2018 年 10 月 19 日 —— 自古逢秋悲寂寥，我言秋日胜春朝