设两次行走等长距离，到达（x1，y1）和（x2，y2）处取得的最大价值为h[x1][y1][x2][y2]。容易证明其具有最优子结构性质。动态规划算法如下。(The maximum value of reaching (x1, Y1) and (X2, Y2) at the same length of two times is h[x1][y1][x2][y2]. It is easy to prove that it has the optimal)