Add Heap.

Author: trekhleb

README.md

@@ -8,6 +8,7 @@
 2. [Queue](https://github.com/trekhleb/javascript-algorithms/tree/master/src/data-structures/queue)
 3. [Stack](https://github.com/trekhleb/javascript-algorithms/tree/master/src/data-structures/stack)
 4. [Hash Table](https://github.com/trekhleb/javascript-algorithms/tree/master/src/data-structures/hash-table)
+5. [Heap](https://github.com/trekhleb/javascript-algorithms/tree/master/src/data-structures/heap)
 
 ## [Algorithms](https://github.com/trekhleb/javascript-algorithms/tree/master/src/algorithms)
 

src/data-structures/heap/MinHeap.js

@@ -0,0 +1,118 @@
+export default class MinHeap {
+  constructor() {
+    // Array representation of the heap.
+    this.heapContainer = [];
+  }
+
+  static getLeftChildIndex(parentIndex) {
+    return (2 * parentIndex) + 1;
+  }
+
+  static getRightChildIndex(parentIndex) {
+    return (2 * parentIndex) + 2;
+  }
+
+  static getParentIndex(childIndex) {
+    return Math.floor((childIndex - 1) / 2);
+  }
+
+  static hasParent(childIndex) {
+    return this.getParentIndex(childIndex) >= 0;
+  }
+
+  hasLeftChild(parentIndex) {
+    return MinHeap.getLeftChildIndex(parentIndex) < this.heapContainer.length;
+  }
+
+  hasRightChild(parentIndex) {
+    return MinHeap.getRightChildIndex(parentIndex) < this.heapContainer.length;
+  }
+
+  leftChild(parentIndex) {
+    return this.heapContainer[MinHeap.getLeftChildIndex(parentIndex)];
+  }
+
+  rightChild(parentIndex) {
+    return this.heapContainer[MinHeap.getRightChildIndex(parentIndex)];
+  }
+
+  parent(childIndex) {
+    return this.heapContainer[MinHeap.getParentIndex(childIndex)];
+  }
+
+  swap(indexOne, indexTwo) {
+    const tmp = this.heapContainer[indexTwo];
+    this.heapContainer[indexTwo] = this.heapContainer[indexOne];
+    this.heapContainer[indexOne] = tmp;
+  }
+
+  peek() {
+    if (this.heapContainer.length === 0) {
+      return null;
+    }
+
+    return this.heapContainer[0];
+  }
+
+  poll() {
+    if (this.heapContainer.length === 0) {
+      return null;
+    }
+
+    if (this.heapContainer.length === 1) {
+      return this.heapContainer.pop();
+    }
+
+    const item = this.heapContainer[0];
+
+    // Move the last element from the end to the head.
+    this.heapContainer[0] = this.heapContainer.pop();
+    this.heapifyDown();
+
+    return item;
+  }
+
+  add(item) {
+    this.heapContainer.push(item);
+    this.heapifyUp();
+  }
+
+  heapifyUp() {
+    let currentIndex = this.heapContainer.length - 1;
+
+    while (
+      MinHeap.hasParent(currentIndex) &&
+      this.parent(currentIndex) > this.heapContainer[currentIndex]
+    ) {
+      this.swap(currentIndex, MinHeap.getParentIndex(currentIndex));
+      currentIndex = MinHeap.getParentIndex(currentIndex);
+    }
+  }
+
+  heapifyDown() {
+    let currentIndex = 0;
+    let nextIndex = 0;
+
+    while (this.hasLeftChild(currentIndex)) {
+      if (
+        this.hasRightChild(currentIndex) &&
+        this.leftChild(currentIndex) > this.rightChild(currentIndex)
+      ) {
+        nextIndex = MinHeap.getRightChildIndex(currentIndex);
+      } else {
+        nextIndex = MinHeap.getLeftChildIndex(currentIndex);
+      }
+
+      if (this.heapContainer[currentIndex] < this.heapContainer[nextIndex]) {
+        break;
+      }
+
+      this.swap(currentIndex, nextIndex);
+      currentIndex = nextIndex;
+    }
+  }
+
+  toString() {
+    return this.heapContainer.toString();
+  }
+}

src/data-structures/heap/README.md

@@ -0,0 +1 @@
+# Heap

src/data-structures/heap/__test__/MinHeap.test.js

@@ -0,0 +1,56 @@
+import MinHeap from '../MinHeap';
+
+describe('MinHeap', () => {
+  it('should create an empty min heap', () => {
+    const minHeap = new MinHeap();
+
+    expect(minHeap).toBeDefined();
+    expect(minHeap.peek()).toBeNull();
+  });
+
+  it('should add items to the heap and heapify it up', () => {
+    const minHeap = new MinHeap();
+
+    minHeap.add(5);
+    expect(minHeap.peek()).toBe(5);
+    expect(minHeap.toString()).toBe('5');
+
+    minHeap.add(3);
+    expect(minHeap.peek()).toBe(3);
+    expect(minHeap.toString()).toBe('3,5');
+
+    minHeap.add(10);
+    expect(minHeap.peek()).toBe(3);
+    expect(minHeap.toString()).toBe('3,5,10');
+
+    minHeap.add(1);
+    expect(minHeap.peek()).toBe(1);
+    expect(minHeap.toString()).toBe('1,3,10,5');
+  });
+
+  it('should poll items from the heap and heapify it down', () => {
+    const minHeap = new MinHeap();
+
+    minHeap.add(5);
+    minHeap.add(3);
+    minHeap.add(10);
+    minHeap.add(1);
+
+    expect(minHeap.toString()).toBe('1,3,10,5');
+
+    expect(minHeap.poll()).toBe(1);
+    expect(minHeap.toString()).toBe('3,5,10');
+
+    expect(minHeap.poll()).toBe(3);
+    expect(minHeap.toString()).toBe('5,10');
+
+    expect(minHeap.poll()).toBe(5);
+    expect(minHeap.toString()).toBe('10');
+
+    expect(minHeap.poll()).toBe(10);
+    expect(minHeap.toString()).toBe('');
+
+    expect(minHeap.poll()).toBeNull();
+    expect(minHeap.toString()).toBe('');
+  });
+});