複選題
(3) In a stack structure, let u denote a push operation and o denote a pop operation. After a sequence
of stack operations, consisting of 3 pushes and 3 pops, an input sequence 123 may change its
order. For example, after uouuoo is performed, 123 will become 132. Which sequence(s) cannot
be obtained from input 123 by stack operations?
(A) 213
(B) 231
(C) 312
(D) 321.
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