電話號碼字母組合

給定一個僅包含數字2-9的字串，返回所有它能表示的字母組合。答案可以按 任意順序 返回。

給出數字到字母的對映如下（與電話按鍵相同）。注意 1 不對應任何字母。

示例 1：

輸入：digits = "23"
輸出：["ad","ae","af","bd","be","bf","cd","ce","cf"]
示例 2：

輸入：digits = ""
輸出：[]
示例 3：

輸入：digits = "2"
輸出：["a","b","c"]

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);
            }
        }

    }
}

　解題詳細步驟　電話號碼的字母組合 - 電話號碼的字母組合 - 力扣（LeetCode） (leetcode-cn.com)