Method set

Data Structures/Persistent UnionFind.cpp:43–47

    return root.size() - 1;
  }
  void set(int r, int i, T x) {
    int k = upd(r, i, x);
    root[r] = root[k];
    root.pop_back();
  }
  node* upd(node* pre, int i, T x, int l, int r) {
    node* cur = new node();
    if (l == r){