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/**

* https://www.topcoder.com/community/competitive-programming/tutorials/binary-indexed-trees/

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

*/

public class BinaryIndexedTree {

private int[] tree;

private int N;

public BinaryIndexedTree(int[] nums) {

if (nums == null) return;

this.N = nums.length;

this.tree = new int[N+1];

constructBIT(nums);

}

public BinaryIndexedTree(int N) {

this.N = N;

this.tree = new int[N+1];

}

private void constructBIT(int[] nums) {

int N = nums.length;

for (int i=0; i<N; i++) {

update(i, nums[i]);

}

}

public void update(int i, int delta) {

int k = i + 1;

while (k <= this.N) {

this.tree[k] += delta;

k += lowBit(k);

}

}

public int query(int i) {

int k = i + 1;

int res = 0;

while (k > 0) {

res += this.tree[k];

k -= lowBit(k);

}

return res;

}

private int lowBit(int x) {

return x & (-x);

}

}