added trinary exercise

config.json

@@ -27,6 +27,7 @@
     "atbash-cipher",
     "accumulate",
     "crypto-square"
+    "trinary"
   ],
   "deprecated": [
   ],

trinary/build.gradle

@@ -0,0 +1,11 @@
+apply plugin: "java"
+apply plugin: "eclipse"
+apply plugin: "idea"
+
+repositories {
+  mavenCentral()
+}
+
+dependencies {
+  testCompile "junit:junit:4.10"
+}

trinary/example.java

@@ -0,0 +1,23 @@
+public class Trinary {
+
+    private static final int RADIX = 3;
+
+    public static int toDecimal(String trinary) {
+
+        if (!validTrinary(trinary)) {
+            return 0;
+        }
+        int result = 0;
+        int power = 1;
+        for (int i = trinary.toCharArray().length - 1; i >= 0; i--) {
+            int digit = Character.digit(trinary.charAt(i), RADIX);
+            result += digit * power;
+            power *= RADIX;
+        }
+        return result;
+    }
+
+    private static boolean validTrinary(String trinary) {
+        return trinary.matches("[012]+");
+    }
+}

trinary/src/main/java/Trinary.java

@@ -0,0 +1,2 @@
+public class Trinary {
+}

trinary/src/test/java/TrinaryTest.java

@@ -0,0 +1,25 @@
+import static org.junit.Assert.assertEquals;
+
+import org.junit.Test;
+
+public class TrinaryTest {
+    @Test
+    public void testNonTrinaryCharacterIsZero() {
+        assertEquals(0, Trinary.toDecimal("-"));
+    }
+
+    @Test
+    public void testNonTrinaryNumberIsZero() {
+        assertEquals(0, Trinary.toDecimal("3"));
+    }
+
+    @Test
+    public void testTrinaryWithNonTrinaryIsZero() {
+        assertEquals(0, Trinary.toDecimal("102-12"));
+    }
+
+    @Test
+    public void testTrinary() {
+        assertEquals(302, Trinary.toDecimal("102012"));
+    }
+}