题目描述：

LeetCode 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.

题目大意：

给定一组人的年龄，尝试为每个人匹配朋友，规则如下。

如果出现下列情况之一，则A和B不可以成为朋友：

age[B] <= 0.5 * age[A] + 7

age[B] > age[A]

age[B] > 100 && age[A] < 100

求最多可以匹配多少组朋友关系

解题思路：

观察题设条件 1 <= ages[i] <= 120，年龄的范围很小

统计每一个年龄的人数

遍历每个人，统计符合其年龄条件约束的人数之和

Python代码：

class Solution(object):
    def numFriendRequests(self, ages):
        """
        :type ages: List[int]
        :rtype: int
        """
        cnt = collections.Counter(ages)
        ans = 0
        for age in ages:
            cnt[age] -= 1
            left, right = age / 2 + 8, age
            ans += sum(cnt[age] for age in range(left, right + 1))
            cnt[age] += 1
        return ans

 

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