Method update

code/data-structures/segment_tree.cpp:13–20

      l == r ? node(l,r,a[l]) :
      node(mk(a,2*i+1,l,m),mk(a,2*i+2,m+1,r)); }
  node update(int at, ll v, int i=0) {
    propagate(i);
    int hl = arr[i].l, hr = arr[i].r;
    if (at < hl || hr < at) return arr[i];
    if (hl == at && at == hr) {
      arr[i].update(v); return arr[i]; }
    return arr[i] =
      node(update(at,v,2*i+1),update(at,v,2*i+2)); }
  node query(int l, int r, int i=0) {
    propagate(i);
    int hl = arr[i].l, hr = arr[i].r;