Add Fenwick Tree (#2570)

DataStructures/Trees/FenwickTree.java

@@ -0,0 +1,34 @@
+package DataStructures.Trees;
+
+public class FenwickTree {
+    private int n;
+    private int fen_t[];
+    
+    /* Constructor which takes the size of the array as a parameter */
+    public FenwickTree(int n) {
+        this.n = n;
+        this.fen_t = new int[n + 1];
+    }
+    
+    /* A function which will add the element val at index i*/
+    public void update(int i, int val) {
+        // As index starts from 0, increment the index by 1
+        i += 1;
+        while (i <= n) {
+            fen_t[i] += val;
+            i += i & (-i);
+        }
+    }
+    
+    /* A function which will return the cumulative sum from index 1 to index i*/
+    public int query(int i) {
+        // As index starts from 0, increment the index by 1
+        i += 1;
+        int cumSum = 0;
+        while (i > 0) {
+            cumSum += fen_t[i];
+            i -= i & (-i);
+        }
+        return cumSum;
+    }
+}