Method build

Data Structures/Persistent Array.cpp:11–15

  int n = 1;
  vector<node*> root;
  int build(vector<T> v) {
    while (n < v.size()) n <<= 1;
    root.push_back(build(0, n - 1, v));
    return root.size() - 1;
  }
  node* build(int l, int r, vector<T>& v) {
    node* cur = new node();
    if (l == r) {