题目：

题解：

class Solution {public List<String> letterCombinations(String digits) {List<String> combinations = new ArrayList<String>();if (digits.length() == 0) {return combinations;}Map<Character, String> phoneMap = new HashMap<Character, String>() {{put('2', "abc");put('3', "def");put('4', "ghi");put('5', "jkl");put('6', "mno");put('7', "pqrs");put('8', "tuv");put('9', "wxyz");}};backtrack(combinations, phoneMap, digits, 0, new StringBuffer());return combinations;}public void backtrack(List<String> combinations, Map<Character, String> phoneMap, String digits, int index, StringBuffer combination) {if (index == digits.length()) {combinations.add(combination.toString());} else {char digit = digits.charAt(index);String letters = phoneMap.get(digit);int lettersCount = letters.length();for (int i = 0; i < lettersCount; i++) {combination.append(letters.charAt(i));backtrack(combinations, phoneMap, digits, index + 1, combination);combination.deleteCharAt(index);}}}
}