Add java solution for 13

Author: gavinfish

java/_013RomanToInteger.java

@@ -0,0 +1,36 @@
+import java.util.HashMap;
+import java.util.Map;
+
+/**
+ * Created by drfish on 25/06/2017.
+ */
+public class _013RomanToInteger {
+    public int romanToInt(String s) {
+        Map<Character, Integer> map = new HashMap<Character, Integer>() {{
+            put('M', 1000);
+            put('D', 500);
+            put('C', 100);
+            put('L', 50);
+            put('X', 10);
+            put('V', 5);
+            put('I', 1);
+        }};
+        int result = 0;
+        for (int i = 0; i < s.length(); i++) {
+            if (i > 0 && map.get(s.charAt(i)) > map.get(s.charAt(i - 1))) {
+                result -= map.get(s.charAt(i - 1));
+                result += map.get(s.charAt(i)) - map.get(s.charAt(i - 1));
+            } else {
+                result += map.get(s.charAt(i));
+            }
+        }
+        return result;
+    }
+
+    public static void main(String[] args) {
+        _013RomanToInteger solution = new _013RomanToInteger();
+        assert 12 == solution.romanToInt("XII");
+        assert 21 == solution.romanToInt("XXI");
+        assert 99 == solution.romanToInt("XCIX");
+    }
+}