Ezvor

493. Reverse Pairs

HardArray Preparing judge… LeetCode

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].

 

Example 1:

Input: nums = [1,3,2,3,1]
Output: 2
Explanation: 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
Explanation: 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 * 104
  • -231 <= nums[i] <= 231 - 1
ArrayBinary SearchDivide and ConquerBinary Indexed TreeSegment TreeMerge SortOrdered SetTreap

Hints

Hint 1
Use the merge-sort technique.
Hint 2
Divide the array into two parts and sort them.
Hint 3
For each integer in the first part, count the number of integers that satisfy the condition from the second part. Use the pointer to help you in the counting process.