.()initstack(s) //栈s初始化为空栈
() setnull (exp) //串exp初始化为空串
() ch in opset //判取出字符是否是操作符
() push (sch) //如ch是运算符则入运算符栈s
() sempty (s) //判栈s是否为空
() succ := false //若读出ch是操作数且栈为空则按出错处理
() exp
() ch //若ch是操作数且栈非空则形成部分中缀表达式
() exp
() gettop(s) //取栈顶操作符
() pop(s) //操作符取出后退栈
() sempty(s) //将pre的最后一个字符(操作数)加入到中缀式exp的最后
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