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Flood Fill Algorithm

  • Difficulty Level : Medium
  • Last Updated : 18 Oct, 2021
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Given a 2D screen arr[][] where each arr[i][j] is an integer representing the color of that pixel, also given the location of a pixel (X, Y) and a color C, the task is to replace the color of the given pixel and all the adjacent same-colored pixels with the given color.
Example: 
 

Input: arr[][] = { 
{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, C = 3 
Output: 
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
1 1 1 1 1 3 1 1 
1 1 1 1 1 3 3
Explanation: 
The values in the given 2D screen indicate colors of the pixels. X and Y are coordinates of the brush, C is the color that should replace the previous color on screen[X][Y] and all surrounding pixels with the same color. Hence all the 2 are replaced with 3. 
 

 

BFS Approach: The idea is to use BFS traversal to replace the color with the new color.
 

  • Create an empty queue lets say Q.
  • Push the starting location of the pixel as given in the input and apply replacement color to it.
  • Iterate until Q is not empty and pop the front node (pixel position).
  • Check the pixels adjacent to the current pixel and push into the queue if valid (had not been colored with replacement color and have the same color as the old color).

Below is the implementation of the above approach:
 



Python3




# Python3 implementation of the approach
 
# Function that returns true if
# the given pixel is valid
def isValid(screen, m, n, x, y, prevC, newC):
    if x<0 or x>= m\
       or y<0 or y>= n or\
       screen[x][y]!= prevC\
       or screen[x][y]== newC:
        return False
    return True
 
 
# FloodFill function
def floodFill(screen, 
            m, n, x, 
            y, prevC, newC):
    queue = []
     
    # Append the position of starting
    # pixel of the component
    queue.append([x, y])
 
    # Color the pixel with the new color
    screen[x][y] = newC
 
    # While the queue is not empty i.e. the
    # whole component having prevC color
    # is not colored with newC color
    while queue:
         
        # Dequeue the front node
        currPixel = queue.pop()
         
        posX = currPixel[0]
        posY = currPixel[1]
         
        # Check if the adjacent
        # pixels are valid
        if isValid(screen, m, n, 
                posX + 1, posY, 
                        prevC, newC):
             
            # Color with newC
            # if valid and enqueue
            screen[posX + 1][posY] = newC
            queue.append([posX + 1, posY])
         
        if isValid(screen, m, n, 
                    posX-1, posY, 
                        prevC, newC):
            screen[posX-1][posY]= newC
            queue.append([posX-1, posY])
         
        if isValid(screen, m, n, 
                posX, posY + 1
                        prevC, newC):
            screen[posX][posY + 1]= newC
            queue.append([posX, posY + 1])
         
        if isValid(screen, m, n, 
                    posX, posY-1
                        prevC, newC):
            screen[posX][posY-1]= newC
            queue.append([posX, posY-1])
 
 
 
# Driver code
screen =[
[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],
    ]
     
# Row of the display
m = len(screen)
 
# Column of the display
n = len(screen[0])
 
# Co-ordinate provided by the user
x = 4
y = 4
 
# Current color at that co-ordinate
prevC = screen[x][y]
 
# New color that has to be filled
newC = 3
 
floodFill(screen, m, n, x, y, prevC, newC)
 
 
# Printing the updated screen
for i in range(m):
    for j in range(n):
        print(screen[i][j], end =' ')
    print()

C#




// C# implementation of the approach
using System;
using System.Collections.Generic;
class GFG {
     
    // Function that returns true if
    // the given pixel is valid
    static bool isValid(int[,] screen, int m, int n, int x, int y, int prevC, int newC)
    {
        if(x < 0 || x >= m || y < 0 || y >= n || screen[x, y] != prevC
           || screen[x,y]== newC)
            return false;
        return true;
    }
  
  
    // FloodFill function
    static void floodFill(int[,] screen, int m, int n, int x, int y, int prevC, int newC)
    {
        List<Tuple<int,int>> queue = new List<Tuple<int,int>>();
  
        // Append the position of starting
        // pixel of the component
        queue.Add(new Tuple<int,int>(x, y));
  
        // Color the pixel with the new color
        screen[x,y] = newC;
  
        // While the queue is not empty i.e. the
        // whole component having prevC color
        // is not colored with newC color
        while(queue.Count > 0)
        {
            // Dequeue the front node
            Tuple<int,int> currPixel = queue[queue.Count - 1];
            queue.RemoveAt(queue.Count - 1);
  
            int posX = currPixel.Item1;
            int posY = currPixel.Item2;
  
            // Check if the adjacent
            // pixels are valid
            if(isValid(screen, m, n, posX + 1, posY, prevC, newC))
            {
                // Color with newC
                // if valid and enqueue
                screen[posX + 1,posY] = newC;
                queue.Add(new Tuple<int,int>(posX + 1, posY));
            }
  
            if(isValid(screen, m, n, posX-1, posY, prevC, newC))
            {
                screen[posX-1,posY]= newC;
                queue.Add(new Tuple<int,int>(posX-1, posY));
            }
  
            if(isValid(screen, m, n, posX, posY + 1, prevC, newC))
            {
                screen[posX,posY + 1]= newC;
                queue.Add(new Tuple<int,int>(posX, posY + 1));
            }
  
            if(isValid(screen, m, n, posX, posY-1, prevC, newC))
            {
                screen[posX,posY-1]= newC;
                queue.Add(new Tuple<int,int>(posX, posY-1));
            }
        }
    }
     
  static void Main() {
    int[,] screen ={
    {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}};
  
    // Row of the display
    int m = screen.GetLength(0);
  
    // Column of the display
    int n = screen.GetLength(1);
  
    // Co-ordinate provided by the user
    int x = 4;
    int y = 4;
  
    // Current color at that co-ordinate
    int prevC = screen[x,y];
  
    // New color that has to be filled
    int newC = 3;
    floodFill(screen, m, n, x, y, prevC, newC);
  
    // Printing the updated screen
    for(int i = 0; i < m; i++)
    {
        for(int j = 0; j < n; j++)
        {
            Console.Write(screen[i,j] + " ");
        }
        Console.WriteLine();
    }
  }
}
 
// This code is contributed by divyeshrabadiya07.

Javascript




<script>
    // Javascript implementation of the approach
     
    // Function that returns true if
    // the given pixel is valid
    function isValid(screen, m, n, x, y, prevC, newC)
    {
        if(x<0 || x>= m || y<0 || y>= n || screen[x][y]!= prevC
           || screen[x][y]== newC)
            return false;
        return true;
    }
 
 
    // FloodFill function
    function floodFill(screen, m, n, x, y, prevC, newC)
    {
        let queue = [];
 
        // Append the position of starting
        // pixel of the component
        queue.push([x, y]);
 
        // Color the pixel with the new color
        screen[x][y] = newC;
 
        // While the queue is not empty i.e. the
        // whole component having prevC color
        // is not colored with newC color
        while(queue.length > 0)
        {
            // Dequeue the front node
            currPixel = queue[queue.length - 1];
            queue.pop();
 
            let posX = currPixel[0];
            let posY = currPixel[1];
 
            // Check if the adjacent
            // pixels are valid
            if(isValid(screen, m, n, posX + 1, posY, prevC, newC))
            {
                // Color with newC
                // if valid and enqueue
                screen[posX + 1][posY] = newC;
                queue.push([posX + 1, posY]);
            }
 
            if(isValid(screen, m, n, posX-1, posY, prevC, newC))
            {
                screen[posX-1][posY]= newC;
                queue.push([posX-1, posY]);
            }
 
            if(isValid(screen, m, n, posX, posY + 1, prevC, newC))
            {
                screen[posX][posY + 1]= newC;
                queue.push([posX, posY + 1]);
            }
 
            if(isValid(screen, m, n, posX, posY-1, prevC, newC))
            {
                screen[posX][posY-1]= newC;
                queue.push([posX, posY-1]);
            }
        }
    }
     
    let screen =[
    [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]];
 
    // Row of the display
    let m = screen.length;
 
    // Column of the display
    let n = screen[0].length;
 
    // Co-ordinate provided by the user
    let x = 4;
    let y = 4;
 
    // Current color at that co-ordinate
    let prevC = screen[x][y];
 
    // New color that has to be filled
    let newC = 3;
 
    floodFill(screen, m, n, x, y, prevC, newC);
 
 
    // Printing the updated screen
    for(let i = 0; i < m; i++)
    {
        for(let j = 0; j < n; j++)
        {
            document.write(screen[i][j] + " ");
        }
        document.write("</br>");
    }
     
    // This code is contributed by divyesh072019.
</script>
Output: 
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

 

DFS Approach: Similarly DFS approach can be used to implement the Flood Fill algorithm as well.
 

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