Add square matrix rotation in-place algorithm.

Author: trekhleb

README.md

@@ -116,6 +116,7 @@ a set of rules that precisely define a sequence of operations.
   * `B` [Tower of Hanoi](src/algorithms/uncategorized/hanoi-tower)
   * `A` [N-Queens Problem](src/algorithms/uncategorized/n-queens)
   * `A` [Knight's Tour](src/algorithms/uncategorized/knight-tour)
+  * `B` [Square Matrix Rotation](src/algorithms/uncategorized/square-matrix-rotation) - in-place algorithm
 
 ### Algorithms by Paradigm
 

src/algorithms/uncategorized/square-matrix-rotation/README.md

@@ -0,0 +1,90 @@
+# Square Matrix In-Place Rotation
+
+## The Problem
+
+You are given an `n x n` 2D matrix (representing an image). 
+Rotate the matrix by `90` degrees (clockwise).
+
+**Note**
+
+You have to rotate the image **in-place**, which means you 
+have to modify the input 2D matrix directly. **DO NOT** allocate
+another 2D matrix and do the rotation.
+
+## Examples
+
+**Example #1**
+
+Given input matrix:
+
+```
+[
+  [1, 2, 3],
+  [4, 5, 6],
+  [7, 8, 9],
+]
+```
+
+Rotate the input matrix in-place such that it becomes:
+
+```
+[
+  [7, 4, 1],
+  [8, 5, 2],
+  [9, 6, 3],
+]
+```
+
+**Example #2**
+
+Given input matrix:
+
+```
+[
+  [5, 1, 9, 11],
+  [2, 4, 8, 10],
+  [13, 3, 6, 7],
+  [15, 14, 12, 16],
+]
+```
+
+Rotate the input matrix in-place such that it becomes:
+
+```
+[
+  [15, 13, 2, 5],
+  [14, 3, 4, 1],
+  [12, 6, 8, 9],
+  [16, 7, 10, 11],
+]
+```
+
+## Algorithm
+
+We would need to do two reflections of the matrix: 
+
+- reflect vertically
+- reflect diagonally from bottom-left to top-right
+
+Or we also could Furthermore, you can reflect diagonally 
+top-left/bottom-right and reflect horizontally.
+
+A common question is how do you even figure out what kind 
+of reflections to do? Simply rip a square piece of paper,
+write a random word on it so you know its rotation. Then,
+flip the square piece of paper around until you figure out
+how to come to the solution.
+ 
+Here is an example of how first line may be rotated using
+diagonal top-right/bottom-left rotation along with horizontal
+rotation.
+
+```
+A B C         A - -         . . A
+/ /     -->   B - -   -->   . . B
+/ . .         C - -         . . C
+```
+
+## References
+
+- [LeetCode](https://leetcode.com/problems/rotate-image/description/)

src/algorithms/uncategorized/square-matrix-rotation/__test__/squareMatrixRotation.test.js

@@ -0,0 +1,59 @@
+import squareMatrixRotation from '../squareMatrixRotation';
+
+describe('squareMatrixRotation', () => {
+  it('should rotate matrix #0 in-place', () => {
+    const matrix = [[1]];
+
+    const rotatedMatrix = [[1]];
+
+    expect(squareMatrixRotation(matrix)).toEqual(rotatedMatrix);
+  });
+
+  it('should rotate matrix #1 in-place', () => {
+    const matrix = [
+      [1, 2],
+      [3, 4],
+    ];
+
+    const rotatedMatrix = [
+      [3, 1],
+      [4, 2],
+    ];
+
+    expect(squareMatrixRotation(matrix)).toEqual(rotatedMatrix);
+  });
+
+  it('should rotate matrix #2 in-place', () => {
+    const matrix = [
+      [1, 2, 3],
+      [4, 5, 6],
+      [7, 8, 9],
+    ];
+
+    const rotatedMatrix = [
+      [7, 4, 1],
+      [8, 5, 2],
+      [9, 6, 3],
+    ];
+
+    expect(squareMatrixRotation(matrix)).toEqual(rotatedMatrix);
+  });
+
+  it('should rotate matrix #3 in-place', () => {
+    const matrix = [
+      [5, 1, 9, 11],
+      [2, 4, 8, 10],
+      [13, 3, 6, 7],
+      [15, 14, 12, 16],
+    ];
+
+    const rotatedMatrix = [
+      [15, 13, 2, 5],
+      [14, 3, 4, 1],
+      [12, 6, 8, 9],
+      [16, 7, 10, 11],
+    ];
+
+    expect(squareMatrixRotation(matrix)).toEqual(rotatedMatrix);
+  });
+});

src/algorithms/uncategorized/square-matrix-rotation/squareMatrixRotation.js

@@ -0,0 +1,28 @@
+/**
+ * @param {*[][]} originalMatrix
+ * @return {*[][]}
+ */
+export default function squareMatrixRotation(originalMatrix) {
+  const matrix = originalMatrix.slice();
+
+  // Do top-right/bottom-left diagonal reflection of the matrix.
+  for (let rowIndex = 0; rowIndex < matrix.length; rowIndex += 1) {
+    for (let columnIndex = rowIndex + 1; columnIndex < matrix.length; columnIndex += 1) {
+      const tmp = matrix[columnIndex][rowIndex];
+      matrix[columnIndex][rowIndex] = matrix[rowIndex][columnIndex];
+      matrix[rowIndex][columnIndex] = tmp;
+    }
+  }
+
+  // Do horizontal reflection of the matrix.
+  for (let rowIndex = 0; rowIndex < matrix.length; rowIndex += 1) {
+    for (let columnIndex = 0; columnIndex < matrix.length / 2; columnIndex += 1) {
+      const mirrorColumnIndex = matrix.length - columnIndex - 1;
+      const tmp = matrix[rowIndex][mirrorColumnIndex];
+      matrix[rowIndex][mirrorColumnIndex] = matrix[rowIndex][columnIndex];
+      matrix[rowIndex][columnIndex] = tmp;
+    }
+  }
+
+  return matrix;
+}