The concepts such as mathematical expression, stack operation and binary tree traversal must be presented in data structure.

数学表达式、栈的操作、二又树的遍历，这几个概念在数据结构的教材中是不可缺少的。

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The realizing recursive algorithm by stack and the Changing recursive algorithm into non-recursive by stack operation are studied. The thinking of recursion and its application are expounded.

分析递归算法的栈实现以及用栈运算转换递归算法为非递归算法，阐述递归思想及其应用。

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If the character is an operator, pop two operands, perform the appropriate operation, and then push the result onto the stack.

如果字符是个操作符，弹出两个操作数，执行恰当操作，然后把结果压入堆栈。

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Phase two of Operation Stack is in operation on the M20, meaning coastbound traffic is taken off the motorway at junction eight and lorries heading for the Channel Tunnel or Port of Dover put back, but held in a queue.

BBC: Kent snow and ice causes disruption

Operation Stack reserves part of the coast-bound motorway for lorries to park before crossing the Channel.

BBC: P&O ferry

Kent Police said they may have to implement Operation Stack on Thursday because of the planned industrial action.

BBC: P&O ferry