Cracking the Coding Interview, 4 Edition - 150 Programming Interview Questions and Solutions

Solutions to Chapter 8 | Recursion
8.6 Implement the “paint fill” function that one might see on many image editing programs.
That is, given a screen (represented by a 2-dimensional array of Colors), a
point, and a new color, fill in the surrounding area until you hit a border of that color.
SOLUTION
pg 64
First, let’s visualize how this method works. When we call Paint Fill (eg, “click” paint fill in the
image editing application) on, say, a green pixel, we want to “bleed” outwards. Pixel by pixel,
we expand outwards calling PaintFill on the surrounding pixel. When we hit a pixel that is
not green, we stop. Surrounding green pixels may still be painted if they are touched by
another Paint Fill operation.
We can implement this algorithm recursively:
1 enum Color {
2 Black, White, Red, Yellow, Green
3 }
4 boolean PaintFill(Color[][] screen, int x, int y, Color ocolor,
5 Color ncolor) {
6 if (x < 0 || x >= screen[0].length ||
7 y < 0 || y >= screen.length) {
8 return false;
9 }
10 if (screen[y][x] == ocolor) {
11 screen[y][x] = ncolor;
12 PaintFill(screen, x - 1, y, ocolor, ncolor); // left
13 PaintFill(screen, x + 1, y, ocolor, ncolor); // right
14 PaintFill(screen, x, y - 1, ocolor, ncolor); // top
15 PaintFill(screen, x, y + 1, ocolor, ncolor); // bottom
16 }
17 return true;
18 }
19
20 boolean PaintFill(Color[][] screen, int x, int y, Color ncolor) {
21 return PaintFill(screen, x, y, screen[y][x], ncolor);
22 }
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Cracking the Coding Interview | Concepts and Algorithms