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

This article is contributed by Anmol. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.

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