Reverse Pairs

arraybinary searchdivide and conquer

Problem Statement

Given an integer array nums , return the number of reverse pairs in the array .

A reverse pair is a pair (i, j) where:

0 <= i < j < nums.length and

nums[i] > 2 * nums[j] .

Examples

Example 1

Input: nums = [1,3,2,3,1]
Output: 2
The reverse pairs are:
(1, 4) --> nums[1] = 3, nums[4] = 1, 3 > 2 * 1
(3, 4) --> nums[3] = 3, nums[4] = 1, 3 > 2 * 1

Example 2

Input: nums = [2,4,3,5,1]
Output: 3
The reverse pairs are:
(1, 4) --> nums[1] = 4, nums[4] = 1, 4 > 2 * 1
(2, 4) --> nums[2] = 3, nums[4] = 1, 3 > 2 * 1
(3, 4) --> nums[3] = 5, nums[4] = 1, 5 > 2 * 1

Constraints

1 <= nums.length <= 5 * 10 ^4

-2 ^31 <= nums[i] <= 2 ^31 - 1

😤

Hard

Difficulty

Topic Info

ModuleDSA

CategoryArray

Sub-topicBinary Search