TopCoder SRM587: ThreeColorabilityEasy

public class ThreeColorabilityEasy {
    int n, m;

    int ToIndex(int row, int col) {
        return row * (m + 1) + col;
    }

    List<int>[] ConstrustGraph(string[] cells) {
        int nodeNum = (n + 1) * (m + 1);
        var graph = Enumerable.Range(0, nodeNum).Select(_ => new List<int>()).ToArray();
        var addEdge = new Func<int, int, int, int, int>((r1, c1, r2, c2) => {
            graph[ToIndex(r1, c1)].Add(ToIndex(r2, c2));
            graph[ToIndex(r2, c2)].Add(ToIndex(r1, c1));
            return 0; // Action is Prohibited Class !!!!!
        });

        for (int i = 0; i <= n; i++) {
            for (int j = 0; j <= m; j++) {
                if (i + 1 <= n) addEdge(i, j, i + 1, j);
                if (j + 1 <= m) addEdge(i, j, i, j + 1);
            }
        }
        for (int i = 0; i < n; i++) {
            for (int j = 0; j < m; j++) {
                if (cells[i][j] == 'N') {
                    addEdge(i, j, i + 1, j + 1);
                } else { // 'Z'
                    addEdge(i + 1, j, i, j + 1);
                }
            }
        }
        return graph;
    }

    public string isColorable(string[] cells) {
        this.n = cells.Length;
        this.m = cells[0].Length;
        var graph = ConstrustGraph(cells);
        var color = new int?[graph.Length];

        var qu = new Queue<int>();

        if (cells[0][0] == 'N') {
            color[ToIndex(0, 0)] = 0;
            color[ToIndex(0, 1)] = color[ToIndex(1, 0)] = 2;
        } else {
            color[ToIndex(0, 1)] = 0;
            color[ToIndex(1, 0)] = 1;
            color[ToIndex(0, 0)] = 2;
        }

        qu.Enqueue(ToIndex(1, 1));

        while (qu.Any()) {
            int now = qu.Dequeue();
            if (color[now].HasValue) continue;
            var adjacentColors = graph[now].Select(idx => color[idx]).Where(c => c != null).Distinct();
            if (adjacentColors.Count() == 3) return "No";
            if (adjacentColors.Count() < 2) continue;
            color[now] = 6 - adjacentColors.Sum();
            foreach (var index in graph[now]) {
                if (!color[index].HasValue)
                    qu.Enqueue(index);
            }
        }

        return "Yes";
    }
}