From facd091b1ec5b66b1b337c3b0b0e67848c649715 Mon Sep 17 00:00:00 2001
From: Ananya Chaurasia <65891617+Ananya88@users.noreply.github.com>
Date: Sat, 15 Aug 2020 15:39:17 +0530
Subject: [PATCH] Create Fenwick tree or Binary indexed tree

---
 algos/Fenwick tree or Binary indexed tree | 75 +++++++++++++++++++++++
 1 file changed, 75 insertions(+)
 create mode 100644 algos/Fenwick tree or Binary indexed tree

diff --git a/algos/Fenwick tree or Binary indexed tree b/algos/Fenwick tree or Binary indexed tree
new file mode 100644
index 0000000..5a12519
--- /dev/null
+++ b/algos/Fenwick tree or Binary indexed tree	
@@ -0,0 +1,75 @@
+#include<bits/stdc++.h> 
+using namespace std; 
+
+int getSum(int BITree[], int index) 
+{ 
+	int sum = 0; // Iniialize result 
+
+	// index in BITree[] is 1 more than the index in arr[] 
+	index = index + 1; 
+
+	// Traverse ancestors of BITree[index] 
+	while (index>0) 
+	{ 
+		// Add current element of BITree to sum 
+		sum += BITree[index]; 
+
+		// Move index to parent node in getSum View 
+		index -= index & (-index); 
+	} 
+	return sum; 
+} 
+
+// Updates a node in Binary Index Tree (BITree) at given index 
+void updateBIT(int BITree[], int n, int index, int val) 
+{ 
+	// index in BITree[] is 1 more than the index in arr[] 
+	index = index + 1; 
+
+	
+	while (index <= n) 
+	{ 
+	// Add 'val' to current node of BI Tree 
+	BITree[index] += val; 
+
+	// Update index to that of parent in update View 
+	index += index & (-index); 
+	} 
+} 
+
+int *constructBITree(int arr[], int n) 
+{ 
+
+	int *BITree = new int[n+1]; 
+	for (int i=1; i<=n; i++) 
+		BITree[i] = 0; 
+
+	// Store the actual values in BITree[] using update() 
+	for (int i=0; i<n; i++) 
+		updateBIT(BITree, n, i, arr[i]); 
+
+	return BITree; 
+} 
+
+//main function
+int main() 
+{ 
+        int n;
+        cin>>n;
+        int freq[n];
+        for(int i=0;i<n;i++){
+          cin>>freq[i];
+          }
+
+	int *BITree = constructBITree(freq, n); 
+	cout << "Sum of elements "
+		<< getSum(BITree, 5)<<'\n'; 
+
+	
+	freq[3] += 6; 
+	updateBIT(BITree, n, 3, 6); //Update BIT for above change in arr[] 
+
+	cout << "Sum of elements (after update) " <<getSum(BITree, 5); 
+
+	return 0; 
+}