Building a segment tree

Building a segment tree.cpp

void buildTree(T arr[], int stIndex, int lo, int hi) {
  if (lo == hi) {
    nodes[stIndex].assignLeaf(arr[lo]);
    return;
  }
  
  int left = 2 * stIndex, right = left + 1, mid = (lo + hi) / 2;
  buildTree(arr, left, lo, mid);
  buildTree(arr, right, mid + 1, hi);
  nodes[stIndex].merge(nodes[left], nodes[right]);
}