# Flood fill Algorithm – how to implement fill() in paint?

In MS-Paint, when we take the brush to a pixel and click, the color of the region of that pixel is replaced with a new selected color. Following is the problem statement to do this task.
Given a 2D screen, location of a pixel in the screen and a color, replace color of the given pixel and all adjacent same colored pixels with the given color.

Example:

```Input:
screen[M][N] = {{1, 1, 1, 1, 1, 1, 1, 1},
{1, 1, 1, 1, 1, 1, 0, 0},
{1, 0, 0, 1, 1, 0, 1, 1},
{1, 2, 2, 2, 2, 0, 1, 0},
{1, 1, 1, 2, 2, 0, 1, 0},
{1, 1, 1, 2, 2, 2, 2, 0},
{1, 1, 1, 1, 1, 2, 1, 1},
{1, 1, 1, 1, 1, 2, 2, 1},
};
x = 4, y = 4, newColor = 3
The values in the given 2D screen indicate colors of the pixels.
x and y are coordinates of the brush, newColor is the color that
should replace the previous color on screen[x][y] and all surrounding
pixels with same color.

Output:
Screen should be changed to following.
screen[M][N] = {{1, 1, 1, 1, 1, 1, 1, 1},
{1, 1, 1, 1, 1, 1, 0, 0},
{1, 0, 0, 1, 1, 0, 1, 1},
{1, 3, 3, 3, 3, 0, 1, 0},
{1, 1, 1, 3, 3, 0, 1, 0},
{1, 1, 1, 3, 3, 3, 3, 0},
{1, 1, 1, 1, 1, 3, 1, 1},
{1, 1, 1, 1, 1, 3, 3, 1},
};
```

## Recommended: Please solve it on “PRACTICE ” first, before moving on to the solution.

Flood Fill Algorithm:
The idea is simple, we first replace the color of current pixel, then recur for 4 surrounding points. The following is detailed algorithm.

```// A recursive function to replace previous color 'prevC' at  '(x, y)'
// and all surrounding pixels of (x, y) with new color 'newC' and
floodFil(screen[M][N], x, y, prevC, newC)
1) If x or y is outside the screen, then return.
2) If color of screen[x][y] is not same as prevC, then return
3) Recur for north, south, east and west.
floodFillUtil(screen, x+1, y, prevC, newC);
floodFillUtil(screen, x-1, y, prevC, newC);
floodFillUtil(screen, x, y+1, prevC, newC);
floodFillUtil(screen, x, y-1, prevC, newC); ```

The following is C++ implementation of above algorithm.

```// A C++ program to implement flood fill algorithm
#include<iostream>
using namespace std;

// Dimentions of paint screen
#define M 8
#define N 8

// A recursive function to replace previous color 'prevC' at  '(x, y)'
// and all surrounding pixels of (x, y) with new color 'newC' and
void floodFillUtil(int screen[][N], int x, int y, int prevC, int newC)
{
// Base cases
if (x < 0 || x >= M || y < 0 || y >= N)
return;
if (screen[x][y] != prevC)
return;

// Replace the color at (x, y)
screen[x][y] = newC;

// Recur for north, east, south and west
floodFillUtil(screen, x+1, y, prevC, newC);
floodFillUtil(screen, x-1, y, prevC, newC);
floodFillUtil(screen, x, y+1, prevC, newC);
floodFillUtil(screen, x, y-1, prevC, newC);
}

// It mainly finds the previous color on (x, y) and
// calls floodFillUtil()
void floodFill(int screen[][N], int x, int y, int newC)
{
int prevC = screen[x][y];
floodFillUtil(screen, x, y, prevC, newC);
}

// Driver program to test above function
int main()
{
int screen[M][N] = {{1, 1, 1, 1, 1, 1, 1, 1},
{1, 1, 1, 1, 1, 1, 0, 0},
{1, 0, 0, 1, 1, 0, 1, 1},
{1, 2, 2, 2, 2, 0, 1, 0},
{1, 1, 1, 2, 2, 0, 1, 0},
{1, 1, 1, 2, 2, 2, 2, 0},
{1, 1, 1, 1, 1, 2, 1, 1},
{1, 1, 1, 1, 1, 2, 2, 1},
};
int x = 4, y = 4, newC = 3;
floodFill(screen, x, y, newC);

cout << "Updated screen after call to floodFill: n";
for (int i=0; i<M; i++)
{
for (int j=0; j<N; j++)
cout << screen[i][j] << " ";
cout << endl;
}
}
```

Output:

```Updated screen after call to floodFill:
1 1 1 1 1 1 1 1
1 1 1 1 1 1 0 0
1 0 0 1 1 0 1 1
1 3 3 3 3 0 1 0
1 1 1 3 3 0 1 0
1 1 1 3 3 3 3 0
1 1 1 1 1 3 1 1
1 1 1 1 1 3 3 1```

References:
http://en.wikipedia.org/wiki/Flood_fill

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