Evaluate Reverse Polish Notation: AC

Author: sofkyle

LeetCodePrj/Java/leetcode/medium/page2/EvaluateReversePolishNotation.java

@@ -1,7 +1,32 @@
 package leetcode.medium.page2;
 
+import java.util.Stack;
+
 public class EvaluateReversePolishNotation {
     public int evalRPN(String[] tokens) {
-        return 0;
+        Stack<Integer> stack = new Stack<>();
+
+        for (String c : tokens) {
+            if (c.equals("+") || c.equals("-") || c.equals("*") || c.equals("/")) {
+                int result = 0;
+                Integer a = stack.pop();
+                Integer b = stack.pop();
+                result += calculate(b, a, c);
+                stack.push(result);
+            } else {
+                stack.push(Integer.valueOf(c));
+            }
+        }
+        return stack.pop();
+    }
+
+    private int calculate(int a, int b, String operator) {
+        switch (operator) {
+            case "+": return a + b;
+            case "-": return a - b;
+            case "*": return a * b;
+            case "/": return a / b;
+            default: return 0;
+        }
     }
 }

LeetCodePrj/Java/leetcode/medium/page2/MediumPage2.java

@@ -17,4 +17,10 @@ public void testSortList() {
         ListNode sortedList = sortList.sortList(node);
     }
 
+    @Test
+    public void testEvalRPN() {
+        String[] input = new String[] {"2", "1", "+", "3", "*"};
+        EvaluateReversePolishNotation evalRPN = new EvaluateReversePolishNotation();
+        System.out.println(evalRPN.evalRPN(input));
+    }
 }