diff --git a/montecarlo/fenwicktree.hpp b/montecarlo/fenwicktree.hpp
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+++ b/montecarlo/fenwicktree.hpp
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+// Taken from
+// https://kartikkukreja.wordpress.com/2013/05/11/bit-fenwick-tree-data-structure-c-implementation/
+// Has MIT licence
+// Slightly modified
+#pragma once
+
+#define LSOne(S) (S & (-S))
+
+template <typename T>
+class BIT {
+	T* ft;
+    size_t size;
+public:
+	// initialize a BIT of n elements to zero
+	BIT(size_t n) {
+		size = n;
+		ft = new T[n+1](); // () sets it to zero
+	}
+
+	~BIT()	{
+		delete [] ft;
+	}
+
+	// returns sum of the range [1...b]
+	T sum(size_t b) {
+		T sum = 0;
+		for (; b; b -= LSOne(b)) sum += ft[b];
+		return sum;
+	}
+
+	// returns sum of the range [a...b]
+	T sum(size_t a, size_t b) {
+		return sum(b) - (a == 1 ? 0 : sum(a - 1));
+	}
+
+	// update value of the k-th element by v (v can be +ve/inc or -ve/dec)
+	// i.e., increment or decrement kth element by a value v
+	void update(size_t k, T v) {
+		for (; k <= size; k += LSOne(k)) ft[k] += v;
+	}
+
+    // divide each original frequency by c
+	void scaleDown(T c){
+        for (int i=1 ; i<=size ; i++) ft[i] /= c;
+    }
+
+    // multiply each original frequency by c
+    void scaleUp(T c){
+        for (int i=1 ; i<=size ; i++) ft[i] *= c;
+    }
+};