曾经被某公司面试，发了一道英文算法面试题，让用java写出算法。

Given an integer array containing digits from [0, 9], the task is to print all possible letter combinations that the numbers could represent. A mapping of digit to letters (just like on the telephone buttons) is being followed. Note that 0 and 1 do not map to any letters

int[] sizeTable = {0, 99}; 			private void checkInput(int[] inputArray) throws Exception	{		if(0 == inputArray.length)		{			throw new Exception("input one null array, please check");		}	}		public List<String> convertDigitToTetters(int[] inputArray)	{		List<List<String>> lists = new ArrayList<List<String>>();		List<String> resultList = new ArrayList<>();		try {			checkInput(inputArray);			List<String> subList = null;			for(int x : inputArray)			{				subList = digitSpilt(x);				lists.add(subList);			}									for (List<String> list : lists) {	            if (resultList.isEmpty()) {	            	resultList = list;	            } else {	            	resultList = resultList.stream().flatMap(item -> list.stream().map(item2 -> item + item2)).collect(Collectors.toList());	            }	        }		} catch (Exception e) {			e.printStackTrace();		}finally {			return resultList;		}		}	public List<String> digitSpilt(int digit) throws Exception	{		int max = Arrays.stream(sizeTable).max().getAsInt();		int min = Arrays.stream(sizeTable).min().getAsInt();				if (digit < min || digit > max)		{			throw new Exception("the Value out of range, please check it again.");		}				List<Character> list = String.valueOf(digit).chars().mapToObj(item -> (char)item).collect(Collectors.toList());			List<String> tettleList = new ArrayList<String>();		list.stream().forEach(x -> {			String[] array = DigitEnum.getValue(Character.getNumericValue(x)).split(", ");			tettleList.addAll(Arrays.asList(array));		});				return tettleList;	} 

public enum DigitEnum {	ONE(1, ""), TWO(2, "a, b, c"), THREE(3, "d, e, f"), FOUR(4, "g, h, i"), FIVE(5, "j, k, l"), SIX(6,			"m, n, o"), SEVEN(7, "p, q, r, s"), EIGHT(8, "t, u, v"), NINE(9, "w, x, y, z"), ZERO(0, "");	private int key;	private String value;		DigitEnum(int key, String value) {        this.value = value;        this.key = key;    }	public String getValue() {        return value;    }    public int getKey() {        return key;    }        public static String getValue(int key) {     	        for (DigitEnum d : DigitEnum.values()) {              if (d.getKey() == key) {                  return d.getValue();              }          }          return null;      } }

测试用例：

@Test	public void test1() {		DigitToTetters digitToTetters = new DigitToTetters();		int[] arr = { 2, 3 };		String[] strarra = { "ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf" };		List<String> result = digitToTetters.convertDigitToTetters(arr);		Assert.assertArrayEquals(strarra, result.toArray());	}	@Test	public void test2() {		DigitToTetters digitToTetters = new DigitToTetters();		int[] arr = { 9 };		String[] strarra = { "w", "x", "y", "z" };		List<String> result = digitToTetters.convertDigitToTetters(arr);		Assert.assertArrayEquals(strarra, result.toArray());	}	@Test	public void test3() {		DigitToTetters digitToTetters = new DigitToTetters();		int[] arr = { 1, 0 };		String[] strarra = {""};		List<String> result = digitToTetters.convertDigitToTetters(arr);		Assert.assertArrayEquals(strarra, result.toArray());	}	@Test	public void test4() {		DigitToTetters digitToTetters = new DigitToTetters();		int[] arr = { 1, 9 };		String[] strarra = { "w", "x", "y", "z" };		List<String> result = digitToTetters.convertDigitToTetters(arr);		Assert.assertArrayEquals( strarra, result.toArray());	}	@Test	public void test5() {		DigitToTetters digitToTetters = new DigitToTetters();		int[] arr = { 1, 299 };		List<String> result = digitToTetters.convertDigitToTetters(arr);       boolean isFail = result.isEmpty();	   Assert.assertTrue(isFail);	}