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.

二分，注意下表控制，bisect返回的是给定数插入的位置

from bisect import bisect_left, bisect_right

class Solution(object):
    def numFriendRequests(self, a):
        """
        :type ages: List[int]
        :rtype: int
        """
        a.sort()
        res=0
        for i,v in enumerate(a):
            if 0.5*v+7>v: continue
            s = bisect_right(a, 0.5*v+7)
            t = bisect_right(a, v)
            res += (t-s-1) if s<=i<=t else (t-s)
        return res