493. Reverse Pairs
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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.lengthandnums[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
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.