fenwick tree c++ code example
Example: dfenwick tree code c++
// C++ code to demonstrate operations of Binary Index Tree
#include <iostream>
using namespace std;
/* n --> No. of elements present in input array.
BITree[0..n] --> Array that represents Binary Indexed Tree.
arr[0..n-1] --> Input array for which prefix sum is evaluated. */
// Returns sum of arr[0..index]. This function assumes
// that the array is preprocessed and partial sums of
// array elements are stored in BITree[].
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
// in BITree. The given value 'val' is added to BITree[i] and
// all of its ancestors in tree.
void updateBIT(int BITree[], int n, int index, int val)
{
// index in BITree[] is 1 more than the index in arr[]
index = index + 1;
// Traverse all ancestors and add 'val'
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);
}
}
// Constructs and returns a Binary Indexed Tree for given
// array of size n.
int *constructBITree(int arr[], int n)
{
// Create and initialize BITree[] as 0
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]);
// Uncomment below lines to see contents of BITree[]
//for (int i=1; i<=n; i++)
// cout << BITree[i] << " ";
return BITree;
}
// Driver program to test above functions
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 = constructBITree(freq, n);
cout << "Sum of elements in arr[0..5] is "
<< getSum(BITree, 5);
// Let use test the update operation
freq[3] += 6;
updateBIT(BITree, n, 3, 6); //Update BIT for above change in arr[]
cout << "\nSum of elements in arr[0..5] after update is "
<< getSum(BITree, 5);
return 0;
}