每日一练-leetcode*

电话号码字母组合

给定一个仅包含数字 2-9 的字符串，返回所有它能表示的字母组合。答案可以按 任意顺序 返回。

给出数字到字母的映射如下（与电话按键相同）。注意 1 不对应任何字母。

 

 

示例 1：

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

输入：digits = ""
输出：[]
示例 3：

输入：digits = "2"
输出：["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);
            }
        }

    }
}

　解题详细步骤　电话号码的字母组合 - 电话号码的字母组合 - 力扣（LeetCode） (leetcode-cn.com)