Binary Indexed Tree

import java.io.*;
import java.util.*;

class BIT
{
    int arr[];
    
    BIT(int []ar,int len)
    {
         arr=new int[len+1];   
         
         for(int i=0;i<len;i++)
         {
             update(i,len,ar[i]);
         }    
    }
    
    // to get the parent take the binary reperesentation of the child & flip the rightmost set bit 
    
    public void update(int index,int len,int val)
    {
        index++;
        
        while(index<=len)
        {
            arr[index]+=val;
            
            index+=(index)&(-index);                        //first take 2's complement of the number and with the originalnumber
        }                                                   // And then that to the original number to get the next index.                        
    }
    
    
    public int get_sum(int index)
    {
        index++;
        int sum=0;
        
        while(index>0)
        {
            sum+=arr[index];
            index-=(index)&(-index);      //first take 2's complement of the number and with the originalnumber
        }                                 // And then that to the original number to get the next index.            
        
        return sum;
    }    
    
    public void print()
    {
        System.out.println(Arrays.toString(arr));
    }
    
    
    public static void main(String[] args) 
    {
         int ar[] = {1,2,3,4,5};
	 int n = ar.length;
	 BIT tree = new BIT(ar,n);
         tree.update(2,n,-10);
         tree.print();
	 System.out.println(tree.get_sum(2));
    }
}