Added new data structure Fenwick Tree

Fenwick tree is data structure which is basically used to solve range query problems.

Author: nirmalkatiyar

Data_Structure/src/Trees/Fenwick_tree.cpp

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+/*Let, f be some reversible function and A be an array of integers of length N.
+Fenwick tree is a data structure which:
+        1.calculates the value of function f in the given range [l,r] (i.e. f(Al,Al+1,…,Ar)) in O(logn) time;
+        2.updates the value of an element of A in O(logn) time;
+        3.requires O(N) memory, or in other words, exactly the same memory required for A;
+        4.is easy to use and code, especially, in the case of multidimensional arrays.
+Note:- 
+        Fenwick tree is also called Binary Indexed Tree, or just BIT .
+Application:-
+        calculating the sum of a range (i.e. f(A1,A2,…,Ak)=A1+A2+⋯+Ak).  */
+#include <bits/stdc++.h>
+using namespace std;
+
+
+int FNT[1000000] = {0}; //fenwick tree array
+int query(int i)
+{
+        int ans = 0;
+        while (i > 0)
+        {
+                ans += FNT[i];
+                i = i - (i & (-i));
+        }
+        return ans;
+}
+void Build_update(int i, int inc, int N)
+{
+        while (i <= N)
+        {
+                FNT[i] += inc;
+                i += (i & (-i));
+        }
+}
+int main()
+{
+        
+        int n;
+        cin >> n;
+        int a[n];
+        for (int i = 1; i <= n; i++)
+        {
+                cin >> a[i];
+                Build_update(i, a[i], n);
+        }
+        int q, l, r;
+        cin >> q;
+        while (q--)
+        {
+                cin >> l >> r;
+                cout << (query(r) - query(l - 1)) << endl;
+        }
+        return 0;
+}