该问题采用的是动态规划的思想来解题，用备忘录的方法进行递归,递归的式子后面写出，不能直接以汽车行驶的费用为目标来进行动态规划,因为最优子结构性质得不到证明。所以必须把油量和费用一起考虑,作为动态规划的对象,此时就有了最优子结构性质。(The problem is used in the theory of dynamic programming to solve problems, using recursive method to write memos, recursive formula behind, not directly with the automobile driving cost as the goal of dynamic programming, because of the nature of the optimal sub structure can't be proved. Therefore, oil quantity and cost must be considered together, as the object of dynamic programming, it has the optimal substructure property at this time.)