another solution

algorithms/cpp/countOfSmallerNumbersAfterSelf/countOfSmallerNumbersAfterSelf.cpp

@@ -1,5 +1,5 @@
 // Source : https://leetcode.com/problems/count-of-smaller-numbers-after-self/
-// Author : Calinescu Valentin
+// Author : Calinescu Valentin, Hao Chen
 // Date   : 2015-12-08
 
 /*************************************************************************************** 
@@ -73,3 +73,80 @@ public:
         return sol;
     }
 };
+
+
+/*************************************************************************************** 
+ * Another solution - Binary Search Tree
+ ***************************************************************************************/
+
+
+class BinarySearchTreeNode
+{
+    public:
+        int val;     
+        int less;      // count of members less than val
+        int count;     // count of members equal val
+        BinarySearchTreeNode *left, *right;
+        BinarySearchTreeNode(int value) : val(value), less(0),count(1),left(NULL), right(NULL) {}
+};
+
+class BinarySearchTree
+{
+    private:
+        BinarySearchTreeNode* root;
+    public:
+        BinarySearchTree(const int value):root(new BinarySearchTreeNode(value)){ }
+        ~BinarySearchTree() {
+            freeTree(root);
+        }
+        void insert(const int value, int &numLessThan) {
+            insert(root, value, numLessThan);
+        }
+    private:
+        void freeTree(BinarySearchTreeNode* root){
+            if (root == NULL) return;
+            if (root->left) freeTree(root->left);
+            if (root->right) freeTree(root->right);
+            delete root;
+        }
+
+        void insert(BinarySearchTreeNode* root, const int value, int &numLessThan) {
+
+            if(value < root->val) {  // left
+                root->less++;
+                if(root->left == NULL) {
+                    root->left = new BinarySearchTreeNode(value);
+                }else{
+                    this->insert(root->left, value, numLessThan);
+                }
+            } else if(value > root->val) { // right
+                numLessThan += root->less + root->count;
+                if(!root->right) {
+                    root->right = new BinarySearchTreeNode(value);
+                }else{
+                    this->insert(root->right, value, numLessThan);
+                }
+            } else {
+                numLessThan += root->less;
+                root->count++;
+                return;
+            }
+        }
+};
+
+class Solution {
+    public:
+        vector<int> countSmaller(vector<int>& nums) {
+            vector<int> counts(nums.size());
+            if(nums.size() == 0) return counts;
+
+            BinarySearchTree tree(nums[nums.size()-1]);
+
+            for(int i = nums.size() - 2; i >= 0; i--) {
+                int numLessThan = 0;
+                tree.insert( nums[i], numLessThan);
+                counts[i] = numLessThan;
+            }
+            return counts; 
+        }
+};