【算法入门】广度/宽度优先搜索(BFS)
https://blog.csdn.net/raphealguo/article/details/7523411

【算法入门】深度优先搜索(DFS)
https://blog.csdn.net/raphealguo/article/details/7560918

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前导知识：队列，栈，HASH表
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