Add Tower of Hanoi.

Author: trekhleb

README.md

@@ -77,6 +77,7 @@
   * [Eulerian Path and Eulerian Circuit](https://github.com/trekhleb/javascript-algorithms/tree/master/src/algorithms/graph/eulerian-path) - Fleury's algorithm
   * [Strongly Connected Components](https://github.com/trekhleb/javascript-algorithms/tree/master/src/algorithms/graph/strongly-connected-components) - Kosaraju's algorithm
 * **Uncategorized**  
+  * [Tower of Hanoi](https://github.com/trekhleb/javascript-algorithms/tree/master/src/algorithms/uncategorized/hanoi-tower)
   * Union-Find
   * Maze
   * Sudoku
@@ -89,6 +90,7 @@
   * [Prim’s Algorithm](https://github.com/trekhleb/javascript-algorithms/tree/master/src/algorithms/graph/prim) - finding Minimum Spanning Tree (MST) for weighted undirected graph
   * [Kruskal’s Algorithm](https://github.com/trekhleb/javascript-algorithms/tree/master/src/algorithms/graph/kruskal) - finding Minimum Spanning Tree (MST) for weighted undirected graph
 * **Divide and Conquer**
+  * [Tower of Hanoi](https://github.com/trekhleb/javascript-algorithms/tree/master/src/algorithms/uncategorized/hanoi-tower)
   * [Euclidean Algorithm](https://github.com/trekhleb/javascript-algorithms/tree/master/src/algorithms/math/euclidean-algorithm) - calculate the Greatest Common Divisor (GCD)
   * [Permutations](https://github.com/trekhleb/javascript-algorithms/tree/master/src/algorithms/sets/permutations) (with and without repetitions)
   * [Combinations](https://github.com/trekhleb/javascript-algorithms/tree/master/src/algorithms/sets/combinations) (with and without repetitions)

src/algorithms/uncategorized/hanoi-tower/README.md

@@ -0,0 +1,29 @@
+# Tower of Hanoi
+
+The Tower of Hanoi (also called the Tower of Brahma or Lucas'
+Tower and sometimes pluralized) is a mathematical game or puzzle. 
+It consists of three rods and a number of disks of different sizes,
+which can slide onto any rod. The puzzle starts with the disks in 
+a neat stack in ascending order of size on one rod, the smallest 
+at the top, thus making a conical shape.
+
+The objective of the puzzle is to move the entire stack to another 
+rod, obeying the following simple rules:
+
+- Only one disk can be moved at a time.
+- Each move consists of taking the upper disk from one of the 
+stacks and placing it on top of another stack or on an empty rod.
+- No disk may be placed on top of a smaller disk.
+
+![Hanoi Tower](https://upload.wikimedia.org/wikipedia/commons/8/8d/Iterative_algorithm_solving_a_6_disks_Tower_of_Hanoi.gif)
+
+Animation of an iterative algorithm solving 6-disk problem
+
+With `3` disks, the puzzle can be solved in `7` moves. The minimal 
+number of moves required to solve a Tower of Hanoi puzzle 
+is `2n − 1`, where `n` is the number of disks.
+
+## References
+
+- [Wikipedia](https://en.wikipedia.org/wiki/Tower_of_Hanoi)
+- [HackerEarth](https://www.hackerearth.com/blog/algorithms/tower-hanoi-recursion-game-algorithm-explained/)

src/algorithms/uncategorized/hanoi-tower/__test__/hanoiTower.test.js

@@ -0,0 +1,39 @@
+import hanoiTower from '../hanoiTower';
+
+describe('hanoiTower', () => {
+  it('should solve tower of hanoi puzzle with 2 discs', () => {
+    const moveCallbackMock = jest.fn();
+
+    hanoiTower(2, moveCallbackMock);
+
+    expect(moveCallbackMock).toHaveBeenCalledTimes(3);
+
+    expect(moveCallbackMock.mock.calls[0][0]).toBe(1);
+    expect(moveCallbackMock.mock.calls[0][1]).toEqual([1, 2]);
+    expect(moveCallbackMock.mock.calls[0][2]).toEqual([]);
+
+    expect(moveCallbackMock.mock.calls[1][0]).toBe(2);
+    expect(moveCallbackMock.mock.calls[1][1]).toEqual([2]);
+    expect(moveCallbackMock.mock.calls[1][2]).toEqual([]);
+
+    expect(moveCallbackMock.mock.calls[2][0]).toBe(1);
+    expect(moveCallbackMock.mock.calls[2][1]).toEqual([1]);
+    expect(moveCallbackMock.mock.calls[2][2]).toEqual([2]);
+  });
+
+  it('should solve tower of hanoi puzzle with 3 discs', () => {
+    const moveCallbackMock = jest.fn();
+
+    hanoiTower(3, moveCallbackMock);
+
+    expect(moveCallbackMock).toHaveBeenCalledTimes(7);
+  });
+
+  it('should solve tower of hanoi puzzle with 6 discs', () => {
+    const moveCallbackMock = jest.fn();
+
+    hanoiTower(6, moveCallbackMock);
+
+    expect(moveCallbackMock).toHaveBeenCalledTimes(63);
+  });
+});

src/algorithms/uncategorized/hanoi-tower/hanoiTower.js

@@ -0,0 +1,94 @@
+import Stack from '../../../data-structures/stack/Stack';
+
+/**
+ * @param {Stack} fromPole
+ * @param {Stack} toPole
+ * @param {function(disc: number, fromPole: number[], toPole: number[])} moveCallback
+ */
+function moveDisc(fromPole, toPole, moveCallback) {
+  moveCallback(fromPole.peek(), fromPole.toArray(), toPole.toArray());
+
+  const disc = fromPole.pop();
+  toPole.push(disc);
+}
+
+/**
+ * @param {number} numberOfDiscs
+ * @param {Stack} fromPole
+ * @param {Stack} withPole
+ * @param {Stack} toPole
+ * @param {function(disc: number, fromPole: number[], toPole: number[])} moveCallback
+ */
+function hanoiTowerRecursive({
+  numberOfDiscs,
+  fromPole,
+  withPole,
+  toPole,
+  moveCallback,
+}) {
+  if (numberOfDiscs === 1) {
+    // Base case with just one disc.
+    moveDisc(fromPole, toPole, moveCallback);
+  } else {
+    // In case if there are more discs then move them recursively.
+
+    // Expose the bottom disc on fromPole stack.
+    hanoiTowerRecursive({
+      numberOfDiscs: numberOfDiscs - 1,
+      fromPole,
+      withPole: toPole,
+      toPole: withPole,
+      moveCallback,
+    });
+
+    // Move the disc that was exposed to its final destination.
+    hanoiTowerRecursive({
+      numberOfDiscs: 1,
+      fromPole,
+      withPole,
+      toPole,
+      moveCallback,
+    });
+
+    // Move temporary tower from auxiliary pole to its final destination.
+    hanoiTowerRecursive({
+      numberOfDiscs: numberOfDiscs - 1,
+      fromPole: withPole,
+      withPole: fromPole,
+      toPole,
+      moveCallback,
+    });
+  }
+}
+
+/**
+ * @param {number} numberOfDiscs
+ * @param {function(disc: number, fromPole: number[], toPole: number[])} moveCallback
+ */
+export default function hanoiTower(numberOfDiscs, moveCallback) {
+  // Each of three poles of Tower of Hanoi puzzle is represented as a stack
+  // that might contain elements (discs). Each disc is represented as a number.
+  // Larger discs have bigger number equivalent.
+
+  // The pole from where the discs should be moved.
+  const fromPole = new Stack();
+
+  // The middle pole that should be used as a helper.
+  const withPole = new Stack();
+
+  // The destination pole where all discs need to be moved.
+  const toPole = new Stack();
+
+  // Let's create the discs and put them to the fromPole.
+  for (let discSize = numberOfDiscs; discSize > 0; discSize -= 1) {
+    fromPole.push(discSize);
+  }
+
+  hanoiTowerRecursive({
+    numberOfDiscs,
+    fromPole,
+    withPole,
+    toPole,
+    moveCallback,
+  });
+}

src/data-structures/stack/Stack.js

@@ -13,7 +13,7 @@ export default class Stack {
   }
 
   /**
-   * @return {LinkedListNode}
+   * @return {*}
    */
   peek() {
     if (!this.linkedList.tail) {