MaxHeapAdhoc.js

/**
 * The minimalistic (ad hoc) version of a MaxHeap data structure that doesn't have
 * external dependencies and that is easy to copy-paste and use during the
 * coding interview if allowed by the interviewer (since many data
 * structures in JS are missing).
 */
class MaxHeapAdhoc {
  constructor(heap = []) {
    this.heap = [];
    heap.forEach(this.add);
  }

  add(num) {
    this.heap.push(num);
    this.heapifyUp();
  }

  peek() {
    return this.heap[0];
  }

  poll() {
    if (this.heap.length === 0) return undefined;
    const top = this.heap[0];
    this.heap[0] = this.heap[this.heap.length - 1];
    this.heap.pop();
    this.heapifyDown();
    return top;
  }

  isEmpty() {
    return this.heap.length === 0;
  }

  toString() {
    return this.heap.join(',');
  }

  heapifyUp() {
    let nodeIndex = this.heap.length - 1;
    while (nodeIndex > 0) {
      const parentIndex = this.getParentIndex(nodeIndex);
      if (this.heap[parentIndex] >= this.heap[nodeIndex]) break;
      this.swap(parentIndex, nodeIndex);
      nodeIndex = parentIndex;
    }
  }

  heapifyDown() {
    let nodeIndex = 0;

    while (
      (
        this.hasLeftChild(nodeIndex) && this.heap[nodeIndex] < this.leftChild(nodeIndex)
      )
      || (
        this.hasRightChild(nodeIndex) && this.heap[nodeIndex] < this.rightChild(nodeIndex)
      )
    ) {
      const leftIndex = this.getLeftChildIndex(nodeIndex);
      const rightIndex = this.getRightChildIndex(nodeIndex);
      const left = this.leftChild(nodeIndex);
      const right = this.rightChild(nodeIndex);

      if (this.hasLeftChild(nodeIndex) && this.hasRightChild(nodeIndex)) {
        if (left >= right) {
          this.swap(leftIndex, nodeIndex);
          nodeIndex = leftIndex;
        } else {
          this.swap(rightIndex, nodeIndex);
          nodeIndex = rightIndex;
        }
      } else if (this.hasLeftChild(nodeIndex)) {
        this.swap(leftIndex, nodeIndex);
        nodeIndex = leftIndex;
      }
    }
  }

  getLeftChildIndex(parentIndex) {
    return (2 * parentIndex) + 1;
  }

  getRightChildIndex(parentIndex) {
    return (2 * parentIndex) + 2;
  }

  getParentIndex(childIndex) {
    return Math.floor((childIndex - 1) / 2);
  }

  hasLeftChild(parentIndex) {
    return this.getLeftChildIndex(parentIndex) < this.heap.length;
  }

  hasRightChild(parentIndex) {
    return this.getRightChildIndex(parentIndex) < this.heap.length;
  }

  leftChild(parentIndex) {
    return this.heap[this.getLeftChildIndex(parentIndex)];
  }

  rightChild(parentIndex) {
    return this.heap[this.getRightChildIndex(parentIndex)];
  }

  swap(indexOne, indexTwo) {
    const tmp = this.heap[indexTwo];
    this.heap[indexTwo] = this.heap[indexOne];
    this.heap[indexOne] = tmp;
  }
}

export default MaxHeapAdhoc;