Method heapify

code/data-structures/heap.cpp:53–54

  }
  int top() { assert(count > 0); return q[0]; }
  void heapify() { for (int i = count - 1; i > 0; i--)
    if (cmp(i, (i - 1) / 2)) swp(i, (i - 1) / 2); }
  void update_key(int n) {
    assert(loc[n] != -1), swim(loc[n]), sink(loc[n]); }
  bool empty() { return count == 0; }