Have you taken the course named Data Structure? Did you pass it? If you do, you should know that a mathematical expression can be expressed as a tree and why. In this problem, you are given some expressions, and you are supposed to draw the tree.
The expressions are composed of these letters:
(1) 'a', 'b', ..., 'z' : means an operand;
(2) '+', '-', '*', '/' : dyadic operator, means plus sign, subtraction sign, multiplication sign and division sign;
(3) '-' : monadic operator, means negative sign;
(4) '(', ')' : used in pairs to alter priority.