题目内容
单选题 If A is the first data element input into a stack followed by B, C, and D, （ ）is the first element to be removed.
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If A is the first data element input into a queue followed by B, C, and D, （）is the first element to be removed.
填空题 If the tree is not empty, the first node is called ____, which has the indegree of ____.
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.
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