Knight Shortest Path

思路：
这道题和search graph node类似，不要一看到shortest 或者 closest 就被吓到了， 然后就开始比较。BFS的优势就在于，搜索过程中，最先到达满足条件的点的path就是最近的。

实现：
因为是path, 所以每访问一个点就要标记true， 不能再访问了。但是其实这种标记方法是有问题的。在当前point 往下走过的标记了true, 但是并不代表其他路径不能进过这个点啊。

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