code: public int removeMin() {
      int x = this.heap.get(0);
      this.heap.set(0, this.heap.back());
      this.indices.set(this.heap.get(0), 0);
      this.indices.set(x, -1);
      this.heap.pop();
      if (this.heap.size() > 1)
        this.percolateDown(0);
      return x;
    }
docs: Removes the minimal element of the heap.
  @return the minimal element of the heap