@@ -139,14 +139,15 @@ private void merge(int[][] arr, Integer[] count, int l, int m, int r) {
 /**
 * https://discuss.leetcode.com/topic/31405/9ms-short-java-bst-solution-get-answer-when-building-bst
 */
-public List<Integer> countSmaller(int[] nums) {
+public List<Integer> countSmaller4(int[] nums) {
 Integer[] ans = new Integer[nums.length];
 Node root = null;
 for (int i = nums.length - 1; i >= 0; i--) {
 root = insert(nums[i], root, ans, i, 0);
 }
 return Arrays.asList(ans);
 }
+
 private Node insert(int num, Node node, Integer[] ans, int i, int preSum) {
 if (node == null) {
 node = new Node(num, 0);
@@ -175,7 +176,7 @@ public Node(int v, int s) {
 /**
 * https://discuss.leetcode.com/topic/31173/my-simple-ac-java-binary-search-code
 */
-public List<Integer> countSmaller(int[] nums) {
+public List<Integer> countSmaller5(int[] nums) {
 Integer[] ans = new Integer[nums.length];
 List<Integer> sorted = new ArrayList<Integer>();
 for (int i = nums.length - 1; i >= 0; i--) {
@@ -185,6 +186,7 @@ public List<Integer> countSmaller(int[] nums) {
 }
 return Arrays.asList(ans);
 }
+
 private int findIndex(List<Integer> sorted, int target) {
 if (sorted.size() == 0) return 0;
 int start = 0;
@@ -203,4 +205,105 @@ private int findIndex(List<Integer> sorted, int target) {
 return end;
 }
 
+
+public List<Integer> countSmaller6(int[] nums) {
+LinkedList<Integer> res = new LinkedList<>();
+if (nums == null || nums.length == 0) return res;
+int N = nums.length;
+int min = Integer.MAX_VALUE;
+int max = Integer.MIN_VALUE;
+for (int n: nums) {
+min = Math.min(min, n);
+}
+int[] ranks = new int[N];
+for (int i = 0; i < N; i++) {
+ranks[i] = nums[i] - min + 1;
+max = Math.max(ranks[i], max);
+}
+
+BinaryIndexedTree bit = new BinaryIndexedTree(max);
+for (int i=ranks.length-1; i>=0 ;i--) {
+res.addFirst(bit.query(ranks[i] - 1));
+bit.update(ranks[i], 1);
+}
+return res;
+}
+
+class BinaryIndexedTree {
+int[] tree;
+int N;
+
+BinaryIndexedTree(int N) {
+this.N = N;
+this.tree = new int[N+1];
+}
+
+void update(int i, int delta) {
+int k = i + 1;
+while (k <= this.N) {
+this.tree[k] += delta;
+k += lowBit(k);
+}
+}
+
+int query(int i) {
+int k = i + 1;
+int res = 0;
+while (k > 0) {
+res += this.tree[k];
+k -= lowBit(k);
+}
+return res;
+}
+
+private int lowBit(int x) {
+return x & (-x);
+}
+}
+
+
+public List<Integer> countSmaller7(int[] nums) {
+LinkedList<Integer> res = new LinkedList<>();
+if (nums == null || nums.length == 0) return res;
+int N = nums.length;
+Node root = new Node(nums[N-1]);
+res.add(0);
+for (int i=N-2; i>=0; i--) {
+res.addFirst(insert(root, nums[i]));
+}
+return res;
+}
+
+private int insert(Node root, int val) {
+if (root.val == val) {
+root.count++;
+return root.leftCount;
+} else if (root.val > val) {
+root.leftCount++;
+if (root.left == null) {
+root.left = new Node(val);
+return 0;
+}
+return insert(root.left, val);
+} else {
+if (root.right == null) {
+root.right = new Node(val);
+return root.count + root.leftCount;
+}
+return root.count + root.leftCount + insert(root.right, val);
+}
+
+}
+
+class Node {
+Node left;
+Node right;
+int val;
+int count = 1;
+int leftCount = 0;
+Node(int v) {
+this.val = v;
+}
+}
+
 }