Method push

Data Structures/Persistent Array.cpp:73–78

    cur++;
  }
  void push(int r, T x) { // push x at the end of the rth queue
    st.upd(r, R[r], x);
    L[cur] = L[r];
    R[cur] = R[r] + 1;
    cur++;
  }
  T pop(int r) { // pops the top element of the rth queue and returns the popped element
    st.upd(r, L[r], 0);
    L[cur] = L[r] + 1;