Flood Fill Algorithm

Flood fill is a process to alter the area in a multi-dimensional array. Take below gif(from wiki) as an example, with flood fill, the middle area is filled with different color.

This algorithm often used in the bucket fill tool in paint programs or games.

In leetcode Flood Fill problem, the area is an image, the problem is to perform a flood fill from the start pixel to pixels(4-directionally) in same color with a new color.

This problem is pretty simple, we could either use breadth-first approach or depth-first approach.

Let first see the breadth-first approach.

function floodFill(image: number[][], sr: number, sc: number, newColor: number): number[][] {
    const oldColor = image[sr][sc];
    if (oldColor === newColor) return image;

    let queue = [[sr, sc]];
    while (true) {
        const pixel = queue.shift();
        if (!pixel) break;

        const [sr, sc] = pixel;
        if (oldColor !== image[sr][sc]) continue;

        image[sr][sc] = newColor;
        if (sr + 1 < image.length) {
            queue.push([sr + 1, sc]);
        }
        if (sc + 1 < image[0].length) {
            queue.push([sr, sc + 1]);
        }
        if (sr - 1 >= 0) {
            queue.push([sr - 1, sc]);
        }
        if (sc - 1 >= 0) {
            queue.push([sr, sc - 1]);
        }
    }

    return image;
};

It is a pretty standard procedure, just use a queue to save the breadth candidates, and fill the cell one by one.

Now let's see the depth-first approach.

function dfs(image: number[][], sr: number, sc: number, oldColor: number, newColor: number) {
    if (image[sr][sc] !== oldColor) return;

    image[sr][sc] = newColor;
    if (sr + 1 < image.length) dfs(image, sr + 1, sc, oldColor, newColor);
    if (sc + 1 < image[0].length) dfs(image, sr, sc + 1, oldColor, newColor);
    if (sr - 1 >= 0) dfs(image, sr - 1, sc, oldColor, newColor);
    if (sc - 1 >= 0) dfs(image, sr, sc - 1, oldColor, newColor);
}

function floodFill(image: number[][], sr: number, sc: number, color: number): number[][] {
    const oldColor = image[sr][sc];
    if (oldColor !== color) {
        dfs(image, sr, sc, oldColor, color);
    }
    return image;
};