EvaluateReversePolishNotation

Change-Id: I371af5e39ef9577537e5118f287d75f3eb003664

Author: applewjg

EvaluateReversePolishNotation.java

@@ -0,0 +1,40 @@
+/*
+ Author:     Andy, nkuwjg@gmail.com
+ Date:       Jan 31, 2015
+ Problem:    Evaluate Reverse Polish Notation
+ Difficulty: Easy
+ Source:     https://oj.leetcode.com/problems/evaluate-reverse-polish-notation/
+ Notes:
+ Evaluate the value of an arithmetic expression in Reverse Polish Notation.
+ Valid operators are +, -, *, /. Each operand may be an integer or another expression.
+ Some examples:
+  ["2", "1", "+", "3", "*"] -> ((2 + 1) * 3) -> 9
+  ["4", "13", "5", "/", "+"] -> (4 + (13 / 5)) -> 6
+ Solution: stack.
+ */
+
+public class Solution {
+    public int evalRPN(String[] tokens) {
+        Stack<Integer> stk = new Stack<Integer>();
+        for (int i = 0; i < tokens.length; ++i) {
+            if ((tokens[i].compareTo("+") != 0) && (tokens[i].compareTo("-") != 0) 
+                    && (tokens[i].compareTo("*") != 0) && (tokens[i].compareTo("/") != 0)) {
+                stk.push(Integer.parseInt(tokens[i]));
+            } else {
+                int op2 = stk.pop();
+                int op1 = stk.pop();
+                int res = 0;
+                if(tokens[i].compareTo("+") == 0)
+                    res = op1 + op2;
+                else if(tokens[i].compareTo("-") == 0)
+                    res = op1 - op2;
+                else if(tokens[i].compareTo("*") == 0)
+                    res = op1 * op2;
+                else if(tokens[i].compareTo("/") == 0)
+                    res = op1 / op2;
+                stk.push(res);
+            }
+        }
+        return stk.pop();
+    }
+}