Coding Exercises Day 9 (Problem #12)

Problem

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?

void rotate90Degrees(byte[][] image) {
  // Run through rows 0 -> n/2 - 1 and rotate all bytes in that row
  for (int i = 0; i < n/2; i++) {
    for (int j = i; j < n - i; j++) {
      rotateByte90Degrees(image, i, j);
    }
  }  
}

// Rotate all four bytes corresponding to x,y.
void rotateByte90Degrees(byte[][] image, int x, int y) {
  int lastIndex = image.length - 1;
  
  byte nextByte = image[y][lastIndex - x];
  
  // Rotation 1
  image[y][lastIndex - x] = image[x][y];
  
  // Store the byte that was overwritten in x,y.
  // We are treating image[x][y] as an additional temp var.
  image[x][y] = nextByte;
  
  // Store the next byte that will be overwritten
  nextByte = image[lastIndex - x][lastIndex - y];
  
  // Rotation 2
  image[lastIndex - x][lastIndex - y] = image[x][y];

  image[x][y] = nextByte;
  nextByte = image[lastIndex - y][x];

  // Rotation 3
  image[lastIndex - y][x] = image[x][y];
  
  // Rotation 4
  image[x][y] = nextByte;
}