private boolean isBalanced() This private method should return true if the infix expression has balanced parentheses, and false otherwise. It does not check for any other kinds of invalid expressions. So, for example, isBalanced() should return true for the invalid infix expression "( 3 ( ( * * 4 ) 8 ) 7 7 ) 6" because the parentheses are balanced, even though other things are a mess. Use the algorithm described in chapter 5 for checking balanced parentheses. This algorithm should use an instance of your LinkedStack class.

private boolean isValid() This private method should return true if the infix expression is valid in all respects, and false otherwise. For example, isValid() should return false for the infix expression "( 3 ( ( * * 4 ) 8 ) 7 7 ) 6", even though the parentheses are balanced. isValid() should check all of the following:

Whether the expression contains illegal characters (the only valid characters are the digits 0 through 9, the 6 operators and parentheses symbols + - * / % ^ ( ), and spaces.

Whether the parentheses are valid (by calling on isBalanced())

Whether there is an improper order of operators and operands. Observe that each

operator must have an operand before it and after it (an operand can be a number, or an expression in parentheses). For example, the following are NOT valid infix expressions:

"*34"

"3 4 +"

"+"

"3 (4 + 5)" // note that this is implied multiplication...something we will consider

NOT valid

"3**4"

"410"

Note that these ARE valid infix expressions:

"3"

"3 * (5)"

"(4 + 9)"

"((4 + 9))"