TopCoder SRM 583: GameOnABoard

public class GameOnABoard {
    static public int[] dx = { 0, 1, 0, -1 };
    static public int[] dy = { 1, 0, -1, 0 };
    static public void Swap<T>(ref T a, ref T b) { T t = a; a = b; b = t; }

    struct Point { public int row, col, step;}

    public int Solve(string[] cost, int si, int sj) {
        int n = cost.Length;
        int m = cost[0].Length;

        Queue<Point> qu = new Queue<Point>();
        Queue<Point> next = new Queue<Point>();
        qu.Enqueue(new Point() { row = si, col = sj, step = cost[si][sj] == '1' ? 1 : 0 });
        int farthest = 0;
        bool[,] visited = new bool[n, m];
        while (qu.Any() || next.Any()) {
            while (qu.Any()) {
                Point now = qu.Dequeue();
                if (visited[now.row, now.col]) continue;
                visited[now.row, now.col] = true;
                farthest = Math.Max(farthest, now.step);
                for (int k = 0; k < 4; k++) {
                    int nr = now.row + dy[k];
                    int nc = now.col + dx[k];
                    if (0 <= nr && nr < n && 0 <= nc && nc < m) {
                        if (!visited[nr, nc]) {
                            int add = (cost[nr][nc] == '1' ? 1 : 0);
                            if (add == 0)
                                qu.Enqueue(new Point() { row = nr, col = nc, step = now.step + add });
                            else
                                next.Enqueue(new Point() { row = nr, col = nc, step = now.step + add });
                        }
                    }
                }
            }
            Swap(ref qu, ref next);
        }
        return farthest;
    }

    public int optimalChoice(string[] cost) {
        int n = cost.Length;
        int m = cost[0].Length;

        int ans = int.MaxValue;

        for (int si = 0; si < n; si++) {
            for (int sj = 0; sj < m; sj++) {
                ans = Math.Min(ans, Solve(cost, si, sj));
            }
        }

        return ans;
    }
}