/**
* Initializes a priority queue from the array of keys. Takes time proportional to the number of
* keys, using sink-based heap construction.
*
* @param keys the array of keys
*/
public MinPQ(Key[] keys) {
N = keys.length;
pq = (Key[]) new Object[keys.length + 1];
for (int i = 0; i < N; i++) pq[i + 1] = keys[i];
for (int k = N / 2; k >= 1; k--) sink(k);
assert isMinHeap();
}
/**
* Removes and returns a smallest key on the priority queue.
*
* @return a smallest key on the priority queue
* @throws java.util.NoSuchElementException if the priority queue is empty
*/
public Key delMin() {
if (isEmpty()) throw new NoSuchElementException("Priority queue underflow");
exch(1, N);
Key min = pq[N--];
sink(1);
pq[N + 1] = null; // avoid loitering and help with garbage collection
if ((N > 0) && (N == (pq.length - 1) / 4)) resize(pq.length / 2);
assert isMinHeap();
return min;
}
