binary indexed tree BIT

Below given code is for implementing binary indexed tree or BIT for range sum and point updates.

for Further details you can read  HERE

// © NISHANT RAJ
// source :-> http://community.topcoder.com/tc?module=Static&d1=tutorials&d2=binaryIndexedTrees
// created :-> 23 / 03 / 2014 ; 04 : 36
#include <cstdio>
#include <cstdlib>
#include <iostream>
#include <map>
#include <string>
#include <cstring>
#include <sstream>
#include <fstream>
#include <climits>
#include <ctime>
#include <algorithm>
#include <bits/stdc++.h>
using namespace std;
int a[100000],cum[100000],res[100000],maxval;
int get_value(int idx) // reading cumulative frequency of index idx O(log(maxval)). 
{
 int sum=0;
 while(idx>0)
 {
  sum+=res[idx];
  idx-=(idx & -idx);
 }
 return sum;
}
void update(int idx,int val)// updating frequency range 
{
 while(idx<=maxval)
 {
  res[idx]+=val;
  idx+=(idx & -idx);
 }
}
int main()
{
 int t;
 cin>>t;
 while(t--)
 {
  int j,idx,val,k;
  cin>>maxval;
  cum[0]=0;
  for(int i=1;i<=maxval;i++)
  {
   cin>>a[i];   //actual frequency given by user.
   cum[i]+=a[i]+cum[i-1];// cumulative frequency array.
  }
  for(int i=1;i<=maxval;i++)
  {
   j= i + 1 - (1<<(__lg(i & -i)));// __lg(num) ->calculates                                                        //the position of MSB
   res[i]=cum[i] - cum[j-1];// calcualting frequency of particular range
 }
  int qury;
  cin>>qury;
  string in,upd="U",find="S";
  while(qury--)
  {
   cin>>in;
   if(in==upd)
   {
    cin>>idx>>val;
    update(idx,val); // takes O(log(maxval)) time.
   }
   if(in==find)
   {
    cin>>j>>k;   // takes 2*O(log(maxval)) time.
    cout<<(get_value(k) - get_value(j-1))<<endl;
   }
  }
 }
 return 0;
}