Add Fenwick Tree algorithm - Gabriel123Duarte

Author: Gabriel123Duarte

include/fenwick_tree.h

@@ -0,0 +1,44 @@
+#ifndef __FENWICK_H__
+#define __FENWICK_H__
+
+#include <vector>
+
+#define LSONE(x) (x & (-x))
+
+class Fenwick
+{
+	private:
+		std::vector<int> fen;
+	public:
+		Fenwick() {}
+
+		// We don't use the index 0
+		Fenwick(int n)
+		{
+			fen.assign(n + 1, 0);
+		}
+
+		// RSQ 1..a
+		int rsq(int a)
+		{
+			int ans = 0;
+			for(; a; a -= LSONE(a))
+				ans += fen[a];
+			return ans;
+		}
+
+		// RSQ a..b
+		inline int rsq(int a, int b)
+		{
+			return rsq(b) - (a == 1 ? 0 : rsq(a - 1));
+		}
+
+		// Update the value of the k-th element by x
+		void update(int k, int x)
+		{
+			for(; k < (int)fen.size(); k += LSONE(k))
+				fen[k] += x;	
+		}
+};
+
+#endif

src/fenwick_tree_demo.cpp

@@ -0,0 +1,16 @@
+#include <cstdio>
+#include "fenwick_tree.h"
+
+int main()
+{
+	Fenwick ft(5);
+
+	ft.update(2, 1);
+	ft.update(4, 10);
+
+	printf("%d\n", ft.rsq(1));
+
+	ft.update(1, 5);	
+	printf("%d\n", ft.rsq(1));
+	return 0;
+}