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2. Leaf node has no children. (True or False) 3. Assume that the order of input elements is :1,2,3,4,5, if we want to get the sequence 32154 at the output of stack S, what we should execute, using the basic operation of the stack to express, is that push(S,1), push(S,2), push(S,3), pop(S), , push(S,5), pop(S), pop(S). 4. The postfix expression is 234+5, the infix should be , the expression result is 5. The maximum number of edges in an acyclic undirected graph with n vertices is ; the edges in a complete undirected graph with n vertices is