Create count-of-smaller-numbers-after-self.java

Author: maiquynhtruong

count-of-smaller-numbers-after-self.java

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+class Solution {
+    int[] smallerRight;
+    public List<Integer> countSmaller(int[] nums) {
+        int n = nums.length;
+        int[] indexes = new int[n];
+        smallerRight = new int[n];
+        for (int i = 0; i < n; i++) {
+            indexes[i] = i;
+        }
+        mergeSort(nums, indexes, 0, n-1);
+        List<Integer> res = new ArrayList<Integer>();
+        for (int i = 0; i < n; i++) {
+            res.add(smallerRight[i]);
+        }
+        return res;
+    }
+    
+    public void mergeSort(int[] nums, int[] indexes, int start, int end) {
+        if (end <= start) return;
+        int mid = (end+start)/2;
+        
+        mergeSort(nums, indexes, start, mid);
+        mergeSort(nums, indexes, mid+1, end);
+        merge(nums, indexes, start, end);
+    }
+    
+    public void merge(int[] nums, int[] indexes, int start, int end) {
+        int[] sortArr = new int[end - start + 1];
+        int sortIndex = 0, rightCount = 0, mid = (start+end)/2, left = start, right = mid+1;
+        
+        while (left <= mid && right <= end) {
+            if (nums[indexes[left]] > nums[indexes[right]]) {
+                rightCount++;
+                sortArr[sortIndex] = indexes[right++];
+            } else {
+                smallerRight[indexes[left]] += rightCount;
+                sortArr[sortIndex] = indexes[left++];
+            }           
+            sortIndex++;
+        }
+        
+        while (right <= end) {
+            sortArr[sortIndex++] = indexes[right++];
+        }
+        while (left <= mid) {
+            smallerRight[indexes[left]] += rightCount;
+            sortArr[sortIndex++] = indexes[left++];
+        }
+        for (int i = start; i <= end; i++) {
+            indexes[i] = sortArr[i-start];
+        }
+        
+    }
+}