间题7： 仅仅具有“最优子结构 能保证贪心算法的正确吗？ 还需要什么条件？
The first key ingredient is the greedy-choice property:we can assemble a globally optimal solution by making locally optimal(greedy)choices.In other words,when we are considering which choice to make,we make the choice that looks best in the current problem,without considering results from subproblems. 问题8： 你能设想一个证明这个特性的基本方法吗？