operator stack 运算符栈
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。
Push the operator onto the stack.
把这个操作符压入(push)堆栈。
When it is an operator, pop two Numbers from the stack, do the calculation, and push back the result.
当它是一个操作符时，从栈中弹出两个数进行运算，并且将运算结果压入栈中。
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