Inserting infix into a binary tree

For example:

(3 + 4) - (1 * 2) + 1

is

Code:

` + `

/ \

- 1

/ \

+ *

/ \ / \

3 4 1 2

precondition:

The infix expression is being entered one character at a time.

The expression is FULLY parenthesized, like (6-4) or (3+(4-2)) or ((2-1)*6) or (((2+1)-4)^2)

postconditions:

Each node will consist of an operator (root) with two children or a digit with no children (BTW no digits ever have children obviously. Children of operators are either digits or a new subtree).

You can use pre-order and post-order traversal to output the tree in prefix and postfix, and use inorder traversal to evaluate the infix expression

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My question is, how do I store the infix expression into a binary tree? I know how to do everything else