---
title: Flood Fill Algorithm
---
## Flood Fill Algorithm

Flood fill is an algorithm mainly used to determine a bounded area connected to a given node in a multi-dimensional array. It is
a close resemblance to the bucket tool in paint programs.

The most approached implementation of the algorithm is a stack-based recursive function, and that's what we're gonna talk about
next.

### How does it work?

The problem is pretty simple and usually follows these steps:

  1. Take the position of the starting point.
  2. Decide whether you want to go in 4 directions (**N, S, W, E**) or 8 directions (**N, S, W, E, NW, NE, SW, SE**).
  3. Choose a replacement color and a target color.
  4. Travel in those directions.
  5. If the tile you land on is a target, replace it with the chosen color.
  6. Repeat 4 and 5 until you've been everywhere within the boundaries.
  
Let's take the following array as an example:

![alt text](https://github.com/firealex2/Codingame/blob/master/small%208%20grid%20paintefffd.png)

The red square is the starting point and the gray squares are the so called walls. 

For further details, here's a piece of code describing the function:


```c++

int wall = -1;

void flood_fill(int pos_x, int pos_y, int target_color, int color)
{
  
   if(a[pos_x][pos_y] == wall || a[pos_x][pos_y] == color) // if there is no wall or if i haven't been there
      return;                                              // already go back
   
   if(a[pos_x][pos_y] != target_color) // if it's not color go back
      return;
   
   a[pos_x][pos_y] = color; // mark the point so that I know if I passed through it. 
   
   flood_fill(pos_x + 1, pos_y, color);  // then i can either go south
   flood_fill(pos_x - 1, pos_y, color);  // or north
   flood_fill(pos_x, pos_y + 1, color);  // or east
   flood_fill(pos_x, pos_y - 1, color);  // or west
   
   return;

}

```

As seen above, my starting point is (4,4). After calling the function for the start coordinates **x = 4** and **y = 4**,
I can start checking if there is no wall or color on the spot. If that is valid i mark the spot with one **"color"** 
and start checking the other adiacent squares.

Going south we will get to point (5,4) and the function runs again.


### Exercise problem

I always considered that solving a (or more) problem/s using a newly learned algorithm is the best way to fully understand
the concept.

So here's one:

**Statement:** 

In a bidimensional array you are given n number of **"islands"**. Try to find the largest area of an island and
the corresponding island number. 0 marks water and any other x between 1 and n marks one square from the surface corresponding
to island x.

**Input**

  * **n** - the number of islands.
  * **l,c** - the dimensions of the matrix.
  * the next **l** lines, **c** numbers giving the **l**th row of the matrix.
  
**Output**
  
  * **i** - the number of the island with the largest area.
  * **A** - the area of the **i**'th island.

**Ex:**

You have the following input:
```c++
2 4 4
0 0 0 1
0 0 1 1
0 0 0 2
2 2 2 2
```
For which you will get island no. 2 as the biggest island with the area of 5 squares.


### Hints

The problem is quite easy, but here are some hints:

    1. Use the flood-fill algorithm whenever you encounter a new island.
    2. As opposed to the sample code, you should go through the area of the island and not on the ocean (0 tiles).