Segment tree for sum of elements in an array

segment_tree_main.cpp

int n; // size of array
int arr[n]; // global array of elements that are provided as input
int tree[4 * n]; // global structure for containing segment tree

// initialize with global array arr
build(1, 1, n);
// query
query(1, 1, n, a, b); // query against range [a, b]
// update
update(1, 1, n, idx, val);