/**
* Parses a postfix expression read from the specified input stream
* and constructs the corresponding expression tree.
* The postfix expression consists of one-letter symbols,
* one-digit numbers, +, -, * and /.
*
* @param resource $in The input stream.
* @return object ExpressionTree An expression tree.
*/
public static function parsePostfix($in)
{
$stack = new StackAsLinkedList();
while (($c = fgetc($in)) != false) {
if (ord('0') <= ord($c) && ord($c) <= ord('9') || ord('a') <= ord($c) && ord($c) <= ord('z') || ord('A') <= ord($c) && ord($c) <= ord('Z')) {
$stack->push(new ExpressionTree($c));
} elseif ($c == '+' || $c == '-' || $c == '*' || $c == '/') {
$result = new ExpressionTree($c);
$result->attachRight($stack->pop());
$result->attachLeft($stack->pop());
$stack->push($result);
}
}
return $stack->pop();
}
