【leetcode17】电话号码的字母组合

一.问题描述

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”]

二.示例代码

static 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");
    }};

    public static void main(String[] args) {
        List<String> result = letterCombinations("23");
        System.out.println(result);
    }

    public static List<String> letterCombinations(String digits) {
        List<String> combinations = new ArrayList<String>();
        if (digits.length() == 0) {
            return combinations;
        }
        backtrack(combinations, digits, 0, new StringBuffer());
        return combinations;
    }

    public static void backtrack(List<String> combinations,
                                 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, digits, index + 1, combination);
                combination.deleteCharAt(index);
            }
        }
    }