avc278 - 1.07 - Javascript (#62)

Added my solution to the Chapter 1 Problem 7 folder

Author: avc278

JavaScript/chapter01/p07_rotate_matrix/avc278.js

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+// Rotate Matrix: Given an image represented by an NxN matrix, where each pixel in the image is 4 bytes, write a method
+// to rotate the image by 90 degrees. Can you do this in place?
+
+const assert = require("assert");
+
+/**
+ * Rotates the inputted 2d array by 90 degrees
+ * @param  {Array.<number[]>} matrix input 2d array to rotate
+ * @return {null}
+ *
+ * rotateMatrix is a void function since we don't actually return anything, but rather mutate the input array.
+ * The trick here is to take the same knowledge of swapping two elements in an array with a temp to do it in place,
+ *
+ *      const swap = (arr, i, j) => {
+ *        const temp = arr[i];
+ *        arr[i] = arr[j];
+ *        arr[j] = temp;
+ *      }
+ *
+ * and apply that to a 2d array. Instead of tracking i and j in one array, we keep track of i, j, M-1-i, and M-1-j.
+ * As we iterate through the array, the positions that we swap shift by one until we finish rotating elements, layer by
+ * layer.
+ *
+ * Runtime: O(N^2)
+ * Space:   O(1)
+ *
+ */
+const rotateMatrix = (matrix) => {
+  if (matrix.length < 2) return;
+
+  const M = matrix.length;
+  for (let i = 0; i < M / 2; i++) {
+    for (let j = i; j < M - i - 1; j++) {
+      const temp = matrix[i][j];
+      // swap i, j, M-1-i, M-1-j
+      matrix[i][j] = matrix[j][M - 1 - i];
+      matrix[j][M - 1 - i] = matrix[M - 1 - i][M - 1 - j];
+      matrix[M - 1 - i][M - 1 - j] = matrix[M - 1 - j][i];
+      matrix[M - 1 - j][i] = temp;
+    }
+  }
+
+  return;
+};
+
+describe(module.filename, () => {
+  it("should not affect a matrix of size 0", () => {
+    const matrix = [];
+
+    rotateMatrix(matrix);
+    assert.deepEqual(matrix, []);
+  });
+  it("should not affect a matrix of size 1", () => {
+    const matrix = [5];
+
+    rotateMatrix(matrix);
+    assert.deepEqual(matrix, [5]);
+  });
+  it("should rotate a 2x2 matrix 90 degrees", () => {
+    const matrix = [
+      [1, 2],
+      [3, 4],
+    ];
+
+    rotateMatrix(matrix);
+    assert.deepEqual(matrix, [
+      [2, 4],
+      [1, 3],
+    ]);
+  });
+  it("should rotate a 3x3 matrix 90 degrees", () => {
+    const matrix = [
+      [1, 2, 3],
+      [4, 5, 6],
+      [7, 8, 9],
+    ];
+
+    rotateMatrix(matrix);
+    assert.deepEqual(matrix, [
+      [3, 6, 9],
+      [2, 5, 8],
+      [1, 4, 7],
+    ]);
+  });
+  it("should rotate a 4x4 matrix 90 degrees", () => {
+    const matrix = [
+      [11, 12, 13, 14],
+      [15, 16, 17, 18],
+      [19, 20, 21, 22],
+      [23, 24, 25, 26],
+    ];
+
+    rotateMatrix(matrix);
+    assert.deepEqual(matrix, [
+      [14, 18, 22, 26],
+      [13, 17, 21, 25],
+      [12, 16, 20, 24],
+      [11, 15, 19, 23],
+    ]);
+  });
+  it("should bring the matrix to its original position after rotating 4 times.", () => {
+    const matrix = [
+      [1, 2, 3],
+      [4, 5, 6],
+      [7, 8, 9],
+    ];
+
+    rotateMatrix(matrix);
+    rotateMatrix(matrix);
+    rotateMatrix(matrix);
+    rotateMatrix(matrix);
+    assert.deepEqual(matrix, [
+      [1, 2, 3],
+      [4, 5, 6],
+      [7, 8, 9],
+    ]);
+  });
+});