填空题 If the tree is not empty, the first node is called ____, which has the indegree of ____.
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填空题 All nodes in the tree, except ____, must have an indegreee of ____.
填空题 A ____in a tree is a node with an outdegree of zero.
填空题 A leaf in a tree is a node with an outdegree of ____.
填空题 Using the depth_first approach ,you traverse a binary tree in three sequences: ____, ____, and ____.