题目来源：http://community.topcoder.com/stat?c=problem_statement&pm=12784

最近由于考研，一个多月没有做TopCoder了，第一次参加Div1，结果第一题都没搞出来，看了社论之后学到了用DFS染色法判断无向图是否是二分图的方法。深刻感觉本人太水了，要加油！

代码如下：

#include <algorithm>
#include <iostream>
#include <sstream>

#include <string>
#include <vector>
#include <stack>
#include <deque>
#include <queue>
#include <set>
#include <map>

#include <cstdio>
#include <cstdlib>
#include <cctype>
#include <cmath>
#include <cstring>

using namespace std;


/*************** Program Begin **********************/
int dx[] = {0, -1, -1, 0, 1, 1}, dy[] = {-1, 0, 1, 1, 0, -1};
class HexagonalBoard {
private:
	vector <string> board;
	int color[50][50];
	int result, N;
public:
	void DFS(int x, int y, int c)
	{
		int nx, ny;
		if ('X' == board[x][y] && -1 == color[x][y]) {
			color[x][y] = c;
			result = max(result, 1);
			for (int i = 0; i < 6; i++) {
				nx = x + dx[i];
				ny = y + dy[i];
				if (nx < 0 || nx > N-1 || ny < 0 || ny > N-1) {
					continue;
				}
				if ('X' == board[nx][ny]) {
					result = max(result, 2);
					DFS(nx, ny, !c);
					if (c == color[nx][ny]) {
						result = 3;
					}
				}
			}
		}
	}

	int minColors(vector <string> board)
	{
		this->board = board;
		memset(color, -1, sizeof(color));
		result = 0;
		N = board.size();
		for (int i = 0; i < N; i++) {
			for (int j = 0; j < N; j++) {
				DFS(i, j, 0);
			}
		}
		return result;
	}
};

/************** Program End ************************/