LintCode 0425 题目，采用递归实现

src/test/java/com/ossez/lang/tutorial/tests/lintcode/LintCode0425LetterCombinationsTest.java

@@ -0,0 +1,77 @@
+package com.ossez.lang.tutorial.tests.lintcode;
+
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.List;
+
+import org.junit.Test;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+/**
+ * <p>
+ * 425
+ * <ul>
+ * <li>@see <a href="https://www.cwiki.us/display/ITCLASSIFICATION/Gray+Code">https://www.cwiki.us/display/ITCLASSIFICATION/Gray+Code</a>
+ * <li>@see<a href=
+ * "https://www.lintcode.com/problem/letter-combinations-of-a-phone-number/description">https://www.lintcode.com/problem/letter-combinations-of-a-phone-number/description</a>
+ * </ul>
+ * </p>
+ * 
+ * @author YuCheng
+ *
+ */
+public class LintCode0425LetterCombinationsTest {
+
+	private final static Logger logger = LoggerFactory.getLogger(LintCode0425LetterCombinationsTest.class);
+
+	/**
+	 * 
+	 */
+	@Test
+	public void testMain() {
+		logger.debug("LetterCombinationsTest");
+		String digits = "23";
+
+		HashMap<String, String> phoneKeyMap = new HashMap<String, String>();
+		phoneKeyMap.put("0", "");
+		phoneKeyMap.put("1", "");
+		phoneKeyMap.put("2", "abc");
+		phoneKeyMap.put("3", "def");
+		phoneKeyMap.put("4", "ghi");
+		phoneKeyMap.put("5", "jkl");
+		phoneKeyMap.put("6", "mno");
+		phoneKeyMap.put("7", "pqrs");
+		phoneKeyMap.put("8", "tuv");
+		phoneKeyMap.put("9", "wxyz");
+
+		List<String> retStatus = new ArrayList<>();
+
+		if (digits != null && digits.length() != 0) {
+			phoneRecursive(digits, retStatus, phoneKeyMap, "", 0);
+		}
+
+		System.out.println(retStatus);
+	}
+
+	/**
+	 * phoneRecursive
+	 * 
+	 * @param digits
+	 * @param retStatus
+	 * @param phoneKeyMap
+	 * @param comb
+	 * @param index
+	 */
+	private void phoneRecursive(String digits, List<String> retStatus, HashMap<String, String> phoneKeyMap, String comb, int index) {
+		if (index == digits.length()) {
+			retStatus.add(comb);
+			return;
+		}
+
+		char pos = digits.charAt(index);
+		for (char c : ((String) phoneKeyMap.get(String.valueOf(pos))).toCharArray()) {
+			phoneRecursive(digits, retStatus, phoneKeyMap, comb + c, index + 1);
+		}
+	}
+}