[leetcode#150] Evaluate Reverse Polish Notation

problem:

Evaluate the value of an arithmetic expression in Reverse Polish Notation.

Valid operators is + , - , * , / . Each operand is an integer or another expression.

Some Examples:

  ["2", "1", "+", "3", "*")--((2 + 1) (3)-9  ["4", "", "5", "/", "+"], 4 + (13/5))

Analysis:

Reference:https://en.wikipedia.org/wiki/reverse_polish_notationAfter reading the wiki page of the RPN, you could find out ThisProblem isSuperEasy!!!!But I still stupidly make A mistake!!! -----------------------------------------------------------------------------------------mistake1: Forget to take care of the order, when numbers were poped out from stack.|num2| | num1|note:num2 was poped before NUM1!!!For an Oprand (+,-,*,/), we hope num1 (operand) num2i Ingore The order at first, thus I has implemented following wrong codes:if(Token.equals ("+") | | token.equals ("-") | | token.equals ("*") | | token.equals ("/")) {    intNUM1 =integer.valueof (Stack.pop ()); intnum2 =integer.valueof (Stack.pop ()); intNUM3 =Compute (NUM1, num2, token); ...} And it results in following error:input:["4", "3", "-"]output:-1Expected:fix:if(Token.equals ("+") | | token.equals ("-") | | token.equals ("*") | | token.equals ("/")) {    intNUM1 =integer.valueof (Stack.pop ()); intnum2 =integer.valueof (Stack.pop ()); intNUM3 =Compute (num2, NUM1, token); ...}

Solution:

 Public classSolution { Public intEVALRPN (string[] tokens) {if(Tokens = =NULL)            Throw NewIllegalArgumentException ("The passed in tokens array in null"); if(Tokens.length = = 0)            return0; Stack<Integer> stack =NewStack<integer> ();  for(String token:tokens) {if(Token.equals ("+") | | token.equals ("-") | | token.equals ("*") | | token.equals ("/")) {                intNUM1 =integer.valueof (Stack.pop ()); intnum2 =integer.valueof (Stack.pop ()); intNUM3 =Compute (num2, NUM1, token);            Stack.push (NUM3); } Else{                intnum =integer.valueof (token);            Stack.push (num); }        }        returnStack.pop (); }        Private intComputeintNUM1,intnum2, String token) {        intres = 0; Switch(token) { Case"+": Res= Num1 +num2;  Break;  Case"-": Res= NUM1-num2;  Break;  Case"*": Res= NUM1 *num2;  Break;  Case"/": Res= NUM1/num2;  Break; }        returnRes; }}

[leetcode#150] Evaluate Reverse Polish Notation