递归方式建立表达式树，并对表达式树进行递归和非递归两种方式的遍历，遍历方法分为先序、中序和后序，并对表达式树转为后缀表达式求值。-Established recursive expression tree, and the expression tree for recursive and non-recursive traversal of the two methods, divided into first traversal sequence, and after the sequenc