Add 013_roman_to_integer file in java (qiyuangong#54)

* Roman in table

Contributed by @naveen701526

Author: naveen701526

java/013_Roman_to_Integer.java

@@ -0,0 +1,23 @@
+class Solution {
+    public int romanToInt(String s) {
+        int[] arr = new int['A' + 26];
+        arr['I'] = 1;
+        arr['V'] = 5;
+        arr['X'] = 10;
+        arr['L'] = 50;
+        arr['C'] = 100;
+        arr['D'] = 500;
+        arr['M'] = 1000;
+
+        int result = 0;
+        int prev = 0;
+
+        for (int i = s.length() - 1; i >= 0; i--) {
+            int current = arr[s.charAt(i)];
+            result += prev > current ? -current : current;
+            prev = current;
+        }
+
+        return result;
+    }
+}