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493. Reverse Pairs

https://leetcode.com/problems/reverse-pairs/description/

Description

Given an array nums, we call (i, j) an important reverse pair if i < j and nums[i] > 2*nums[j].

You need to return the number of important reverse pairs in the given array.

Example1:
Input: [1,3,2,3,1]
Output: 2

Example2:
Input: [2,4,3,5,1]
Output: 3

Note:

• The length of the given array will not exceed 50,000.
• All the numbers in the input array are in the range of 32-bit integer.

Solution

class Solution(object):
    def reversePairs(self, nums):
        """
        :type nums: List[int]
        :rtype: int
        """
        import bisect, math
        ret = 0
        record = []
        for n in nums:
            v = n*2
            ret += len(record)-bisect.bisect_right(record, v)
            bisect.insort(record, n)
        return ret