Add Leetcode 13. Roman to Integer

src/leetcode/LC_013/Solution13.java

@@ -0,0 +1,42 @@
+package leetcode.LC_013;
+
+import java.util.HashMap;
+import java.util.Map;
+
+/**
+ * 13. Roman to Integer
+ *
+ * I             1
+ * V             5
+ * X             10
+ * L             50
+ * C             100
+ * D             500
+ * M             1000
+ */
+class Solution13 {
+    int romanToInt(String s) {
+        // Initialize a lookup table for each character
+        final Map<Character, Integer> values = new HashMap<>();
+        values.put('I', 1);
+        values.put('V', 5);
+        values.put('X', 10);
+        values.put('L', 50);
+        values.put('C', 100);
+        values.put('D', 500);
+        values.put('M', 1000);
+
+        // Iterate through each char
+        int result = 0;
+        int prev = 0;   // Memorize previous digit's value
+        for (int i = 0; i < s.length(); i++) {
+            final int digit = values.get(s.charAt(i));
+            result += digit;
+            if (digit > prev)
+                result -= prev * 2;
+
+            prev = digit;
+        }
+        return result;
+    }
+}

src/leetcode/LC_013/Solution13Test.java

@@ -0,0 +1,33 @@
+package leetcode.LC_013;
+
+import org.junit.Test;
+
+import static org.junit.Assert.*;
+
+public class Solution13Test {
+
+    @Test
+    public void romanToInt_III() {
+        assertEquals(3, new Solution13().romanToInt("III"));
+    }
+
+    @Test
+    public void romanToInt_IV() {
+        assertEquals(4, new Solution13().romanToInt("IV"));
+    }
+
+    @Test
+    public void romanToInt_IX() {
+        assertEquals(9, new Solution13().romanToInt("IX"));
+    }
+
+    @Test
+    public void romanToInt_LVIII() {
+        assertEquals(58, new Solution13().romanToInt("LVIII"));
+    }
+
+    @Test
+    public void romanToInt_MCMXCIV() {
+        assertEquals(1994, new Solution13().romanToInt("MCMXCIV"));
+    }
+}