segment_tree_build.cpp

#define MX 6

int arr[MX];
int tree[MX*4];

void build(int node, int b, int e) {
  if (b == e) {
    tree[node] = arr[b];
    return;
  }
  
  int left = 2*node;
  int right = 2*node + 1;
  int mid = (b + e)/2;
  
  // left child node
  build(left, b, mid);
  // right child node
  build(right, mid + 1, e);
  // merge the child nodes
  tree[node] = tree[left] + tree[right];
}