Create range-frequency-queries.cpp

Author: kamyu104

C++/range-frequency-queries.cpp

@@ -0,0 +1,21 @@
+// Time:  ctor:  O(n)
+//        query: O(logn)
+// Space: O(n)
+
+class RangeFreqQuery {
+public:
+    RangeFreqQuery(vector<int>& arr) {
+        for (int i = 0; i < size(arr); ++i) {
+            idxs_[arr[i]].emplace_back(i);
+        }
+    }
+    
+    int query(int left, int right, int value) {
+        const auto& arr = idxs_[value];
+        return upper_bound(cbegin(arr), cend(arr), right) -
+               lower_bound(cbegin(arr), cend(arr), left);
+    }
+
+private:
+    unordered_map<int, vector<int>> idxs_;
+};