update

hard/493.reverse-pairs.java

@@ -8,15 +8,25 @@ class Solution {
 
     private int mergeSort(int[] nums, int beg, int end) {
         int res = 0;
-        if (beg <= end) return res;
+        if (end <= beg) return res;
         int mid = beg + (end - beg) / 2;
         res += mergeSort(nums, beg, mid);
         res += mergeSort(nums, mid + 1, end);
         for (int i = beg, j = mid + 1; i <= mid; i++) {
-            for (; j < end && 2 * nums[j] < nums[i]; j++);
-            res += j - mid - 1;
+            for (; j <= end && 2L * nums[j] < (long) nums[i]; j++);
+            res += j - (mid + 1);
         }
-        
+        for (int i = beg, j = mid + 1, k = beg; k <= end; k++) {
+            if (i == mid + 1) {
+                aux[k] = nums[j++];
+            } else if (j == end + 1) {
+                aux[k] = nums[i++];
+            } else {
+                if (nums[i] < nums[j]) aux[k] = nums[i++];
+                else aux[k] = nums[j++];
+            }
+        }
+        System.arraycopy(aux, beg, nums, beg, end - beg + 1);
         return res;
     }
 }