从第一个数字开始遍历其对应的字母，将其加入StringBuffer中，继续深度优先搜索，当访问到最后一个数字的时候，将StringBuffer存储到ans中，然后回溯到下一个对应字母。

class Solution {public List<String> letterCombinations(String digits) {List<String> ans = new ArrayList<>();if (digits.length() == 0) return ans;Map<Character, String> map = new HashMap<>() {{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(ans, digits, map, 0, new StringBuffer());return ans;}public void backtrack(List<String> ans, String digits, Map<Character, String> map, int index, StringBuffer sb) {if (index == digits.length()) ans.add(sb.toString());else {char ch = digits.charAt(index);String letters = map.get(ch);for (int i = 0; i < letters.length(); ++i) {sb.append(letters.charAt(i));backtrack(ans, digits, map, index + 1, sb);sb.deleteCharAt(index);}}}
}

拓展：
StringBuffer中的删除对应字符的方法是deleteCharAt()