CareerCup

FloodFill(Pixel p, Color curr, Color new)
{
if Color(p) != curr then return
Q.Add(p); // Q is a queue that stores pixels
while Q is not empty do
{
p = Q.getFirst();
if Color(p) == curr then set color(p) = new;
Q.Add(p.rightPixel());
Q.Add(p.leftPixel());
Q.Add(p.topPixel());
Q.Add(p.bottomPixel());
}
return;
}

Java implementation. No need for "visited" set because when we color a pixel with the new color we know we've already visited it.

private static void fill(int r, int c, int newColor, int[][] mat) {
    int rows = mat.length;
    if (rows == 0) {
        return;
    }
    int cols = mat[0].length;

    int oldColor = mat[r][c];
    if (oldColor == newColor) {
        return;
    }

    Queue<Integer> queue = new ArrayDeque<>();
    queue.add(r * cols + c);

    while (!queue.isEmpty()) {
        int pos = queue.poll();
        int currR = pos / cols;
        int currC = pos % cols;

        if (mat[currR][currC] != oldColor) {
            continue;
        }

        mat[currR][currC] = newColor;
        if (currC > 0)
            queue.add(currR * cols + currC - 1);
        if (currC < cols - 1)
            queue.add(currR * cols + currC + 1);
        if (currR > 0)
            queue.add((currR - 1) * cols + currC);
        if (currR < rows - 1)
            queue.add((currR + 1) * cols + currC);
    }
}