二叉树实现简易计算器

An arithmetic expression is input by the keyboard and input in the form of an infix. The trial writer converts the infix expression into a binary expression tree, and obtains the value of the calculation expression by the post-order traversal. Basic requirements: a. It is required to judge whether the input expression is legal. It is illegal to have an error message. b. Convert the infix expression to a binary expression tree. c. Post-order traversal to find the value of the expression