05 August 2008
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 Example 2:
Input: nums = [2,4,3,5,1] Output: 3
Constraints:
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2
1 <= nums.length <= 5 * 104
-231 <= nums[i] <= 231 - 1
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class Solution {
public int reversePairs(int[] nums) {
return mergeSort(nums, 0, nums.length-1);
}
public int mergeSort(int[] nums, int l, int r) {
if (l >= r) return 0;
int m = l + (r - l) / 2;
int res = mergeSort(nums, l, m) + mergeSort(nums, m+1, r);
// Classic Merge Sort Approach
int[] tmp = new int[r-l+1];
int i = l, j = m+1, k = 0;
while (i <= m && j <= r) {
if (nums[i] <= nums[j]) tmp[k++] = nums[i++];
else tmp[k++] = nums[j++];
}
while (i <= m) tmp[k++] = nums[i++];
while (j <= r) tmp[k++] = nums[j++];
// Count valid nums[i]
for (i = l, j = m+1; i <= m; i++) {
while (j <= r && nums[i] / 2.0 > nums[j]) j++;
res += j - m - 1;
}
// Applying the sorted results to nums
// Apply only after counting is because this would
// mess up the relative positions of left & right sections
for (i = l, j = 0; i <= r; i++, j++) nums[i] = tmp[j];
return res;
}
}