Flood-Fill Algorithm using JavaScript

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: 

const grid = [
     [1, 1, 0, 0, 0],
     [1, 1, 0, 1, 1],
     [0, 0, 0, 1, 0],
     [0, 0, 1, 1, 0],
     [1, 0, 0, 0, 1]
];

Starting point: (0,0)
TargetColor: 2

Output:

2 2 0 0 0
2 2 0 1 1
0 0 0 1 0
0 0 1 1 0
1 0 0 0 1

Explanation:

In the 2D grid, each value indicates the color of a pixel. 
The starting point (0,0) indicates the coordinates from where 
we begin the flood-fill. We replace all connected pixels 
with the target color (2). So, all connected '1's starting 
from the starting point are replaced with '2' in the grid.

Using DFS

Using Depth-First Search (DFS), the flood-fill algorithm recursively explores neighboring pixels from a starting point (x, y).

Approach:

• Start at the given starting point (x, y) in the grid.
• Check if the current pixel is within the grid boundaries and has the same color as the starting pixel.
• If the current pixel is valid, replace its color with the target color.
• Recursively explore the neighboring pixels of the current pixel.
• For each neighboring pixel:
  • Check if it’s within the grid boundaries and has the same color as the starting pixel.
  • If valid, apply the replacement color and recursively explore its neighbors.
• Repeat steps 4-5 until all connected pixels with the same color as the starting pixel are replaced with the target color.

Example: Implementation to implement Flood-fill Algorithm using DFS.

JavaScript

function floodFillRecursive(grid, x, y, targetColor, startColor) {
    if (x < 0 || x >= grid.length || y < 0
        || y >= grid[0].length ||
        grid[x][y] !== startColor) {
        return;
    }

    grid[x][y] = targetColor;

    floodFillRecursive(grid, x + 1, y, targetColor, startColor);
    floodFillRecursive(grid, x - 1, y, targetColor, startColor);
    floodFillRecursive(grid, x, y + 1, targetColor, startColor);
    floodFillRecursive(grid, x, y - 1, targetColor, startColor);
}

// Example usage
const grid = [
    [1, 1, 0, 0, 0],
    [1, 1, 0, 1, 1],
    [0, 0, 0, 1, 0],
    [0, 0, 1, 1, 0],
    [1, 0, 0, 0, 1]
];

const startX = 0;
const startY = 0;
const targetColor = 2;

floodFillRecursive(grid, startX, startY, targetColor, grid[startX][startY]);

console.log("Output Using DFS:");
console.log(grid);

Output

Output Using DFS:
[
  [ 2, 2, 0, 0, 0 ],
  [ 2, 2, 0, 1, 1 ],
  [ 0, 0, 0, 1, 0 ],
  [ 0, 0, 1, 1, 0 ],
  [ 1, 0, 0, 0, 1 ]
]

Time complexity: O(m*n)

Space Complexity: O(m+n), due to recursive call stack.

Using BFS

It explores neighboring pixels iteratively from a starting point in the grid.

Approach:

• Initialize a queue (Q).
• Push the starting location of the pixel as given in the input and apply the replacement color to it.
• Iterate over the queue until it’s not empty.
• Pop the front node from the queue.
• Check the adjacent pixels of the current pixel.
• If adjacent pixels are valid (within grid boundaries and have the same color as the starting pixel), push them into the queue and apply the replacement color.
• Repeat steps 4-6 until all connected pixels with the same color as the starting pixel are replaced with the target color.

Example: Implementation to implement Flood-fill Algorithm using DFS.

JavaScript

function floodFillIterative(grid, startX, startY, targetColor) {
    const startColor = grid[startX][startY];
    const queue = [[startX, startY]];
    const directions = [[-1, 0], [1, 0], [0, -1], [0, 1]];

    while (queue.length > 0) {
        const [x, y] = queue.shift();
        grid[x][y] = targetColor;

        for (const [dx, dy] of directions) {
            const newX = x + dx;
            const newY = y + dy;
            if (newX >= 0 && newX < grid.length
                && newY >= 0 && newY < grid[0].length
                && grid[newX][newY] === startColor) {
                queue.push([newX, newY]);
            }
        }
    }
}

// Example usage
const grid = [
    [1, 1, 0, 0, 0],
    [1, 1, 0, 1, 1],
    [0, 0, 0, 1, 0],
    [0, 0, 1, 1, 0],
    [1, 0, 0, 0, 1]
];

const startX = 2;
const startY = 2;
const targetColor = 2;

floodFillIterative(grid, startX, startY, targetColor);

console.log("Output after BFS approach:");
console.log(grid);

Output

Output after BFS approach:
[
  [ 1, 1, 2, 2, 2 ],
  [ 1, 1, 2, 1, 1 ],
  [ 2, 2, 2, 1, 0 ],
  [ 2, 2, 1, 1, 0 ],
  [ 1, 2, 2, 2, 1 ]
]

Time Complexity: O(N*M)

Space Complexity: O(N*M)