1 class Solution {
 2     public int reversePairs(int[] nums) {
 3         return mergeSort(nums, 0, nums.length - 1);
 4     }
 5
 6     public int mergeSort(int[] nums, int start, int end) {
 7         if (start >= end) return 0;
 8         int total =  0;
 9         int mid = start + (end - start) / 2;
10         total += mergeSort(nums, start, mid);
11         total += mergeSort(nums, mid + 1, end);
12         total += merge(nums, start, mid, end);
13         return total;
14     }
15
16     public int merge(int[] nums, int start, int mid, int end) {
17         int[] temp = new int[end - start + 1];
18         int p1 = start, p2 = mid + 1;
19         int index = 0,count = 0;
20
21         while (p1 <= mid && p2 <= end) {
22             if ((long) nums[p1] > 2 * ((long) nums[p2])) {
23                 count += mid - p1 + 1;
24                 p2++;
25             } else {
26                 p1++;
27             }
28         }
29         p1 = start;
30         p2 = mid + 1;
31         while (p1 <= mid && p2 <= end) {
32             if (nums[p1] < nums[p2]) {
33                 temp[index++] = nums[p1++];
34             } else {
35                 temp[index++] = nums[p2++];
36             }
37         }
38
39         while(p1 <= mid) {
40             temp[index++] = nums[p1++];
41         }
42         while(p2 <= end) {
43             temp[index++] = nums[p2++];
44         }
45         System.arraycopy(temp, 0, nums, start, end - start + 1);
46         return count;
47     }
48 }