对四则运算表达式的抽象语法树进行后序遍历，可以得到输入表达式所对应的四元式序列（实验4要用到这样的四元式序列）。例如输入1+2*(3+4)，对应的抽象语法树的中序遍历序列、四元式序列分别为-After the abstract syntax tree arithmetic expressions were preorder, you can get input expression quaternion corresponding sequence (Experiment 4 to use this quaternion sequence). For example, enter 1+2* (3+4), the traversal sequence corresponding abstract syntax tree quaternion sequences were