equation :5+3 PostFix ==> 5 3 +      8.0 Valid

equation :(5+3) PostFix ==>  5 3 +      8.0 Valid

equation :(5-3) PostFix ==>  5 3 -      2.0 Valid

equation :5 PostFix ==> 5     5.0 Valid

equation :5+(3-4) PostFix ==> 5 3 4 -  +      4.0 Valid

equation :7*(9/3) PostFix ==> 7 9 3 /  *      21.0 Valid

equation :9/(5-5) PostFix ==> 9 5 5 -  /      Infinity Valid

equation :5+6*(19-5*2)/3 PostFix ==> 5 6 19 5 2 * -   * 3 /  +      23.0 Valid


(0-5)+3  is invalid 


(7*(9/3)  is invalid 


7*(9/3))  is invalid