Fenwick Tree note

Video by Tushar

https://www.geeksforgeeks.org/binary-indexed-tree-or-fenwick-tree-2/

#include <bits/stdc++.h>
# define LL long long
using namespace std;


void updateTree(int BITree[], int index, int n,int val){
    index++;
    while(index<=n){
        BITree[index]+=val;
        index+=index&(-index);
    }
}

int* constructTree(int arr[], int n){
    int* BITree=new int[n+1];
    for(int i=1;i<=n;++i) BITree[i]=0;

    for(int i=0;i<n;++i){
        updateTree(BITree,i,n,arr[i]);
    }
    return BITree;
}

int getSum(int BITree[], int index){
    index++;
    int res=0;
    while(index>0){
        res+=BITree[index];
        index-=index&(-index);
    }
    return res;
}

int main(){
    int freq[] = {2, 1, 1, 3, 2, 3, 4, 5, 6, 7, 8, 9};
    int n = sizeof(freq)/sizeof(freq[0]);

    int *BITree=constructTree(freq,n);

    int sum=getSum(BITree,5);
    cout<<"the sum from 0 to 5 is "<<sum<<"\n";

    freq[3]+=6;
    updateTree(BITree,3,n,6);
    sum=getSum(BITree,5);
    cout<<"after update, the sum is "<<sum<<"\n";
    return 0;
}