CONCEPT  DEFINE:
                   


formular is two son tree,  has 3 kind of tree node.
1.  operator (including + - * / =)
2.  parameter
3.  number (non zero unsign integer)

leaf node type can be and only can be paramter or number.
operator node must have two sons.

!!!NOTE:
    All operator with same priority. if you want to change the priority, use ().
    zero not support for zero may cause calcuate exception.

example:
1.
    experssion:         2
    formular tree:    (2)

2.
    experssion:         section_lenth
    formular tree:    (section_lenth)

3.
    experssion:         A + 2
    formular tree:    (+)
                        /     \
                      (A)      (2)

4.
    experssion:        3 * A  + (B - 2 ) + C
    formular tree:  
                                     (+) 
                                  /       \
                             (+)           (C)
                          /       \
                      (*)          (-)
                    /     \        /   \
                  (3)      (A) (B)    (2)

5.
    experssion:        3 + A  - B * 2  = 5
    formular tree:  
                                   (=) 
                                /       \
                            (*)          (5)
                          /      \
                       (-)        (2)
                     /     \      
                  (+)      (B)
                /     \      
             (3)      (A)