Merge pull request #1895 from CodingTestStudy2/염혜정

[염혜정] Day25

Author: github-actions[bot]

leetcode3/염혜정/315. Count of Smaller Numbers After Self.java

@@ -0,0 +1,42 @@
+// o(nlogn)
+
+class Solution {
+    public List<Integer> countSmaller(int[] nums) {
+        int n = nums.length;
+        int[] counts = new int[n];
+        int[] indices = new int[n];
+        for (int i = 0; i < n; i++) indices[i] = i;
+        mergeSort(nums, indices, counts, 0, n - 1);
+        List<Integer> result = new ArrayList<>();
+        for (int c : counts) result.add(c);
+        return result;
+    }
+
+    private void mergeSort(int[] nums, int[] indices, int[] counts, int left, int right) {
+        if (left >= right) return;
+        int mid = (left + right) >>> 1;
+        mergeSort(nums, indices, counts, left, mid);
+        mergeSort(nums, indices, counts, mid + 1, right);
+        merge(nums, indices, counts, left, mid, right);
+    }
+
+    private void merge(int[] nums, int[] indices, int[] counts, int left, int mid, int right) {
+        int[] temp = new int[right - left + 1];
+        int i = left, j = mid + 1, k = 0, rightCount = 0;
+        while (i <= mid && j <= right) {
+            if (nums[indices[j]] < nums[indices[i]]) {
+                rightCount++;
+                temp[k++] = indices[j++];
+            } else {
+                counts[indices[i]] += rightCount;
+                temp[k++] = indices[i++];
+            }
+        }
+        while (i <= mid) {
+            counts[indices[i]] += rightCount;
+            temp[k++] = indices[i++];
+        }
+        while (j <= right) temp[k++] = indices[j++];
+        for (int p = 0; p < temp.length; p++) indices[left + p] = temp[p];
+    }
+}