leetcode题目17: 电话号码的组合（java）

题目描述

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。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

输入："23"
输出：["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"].

代码

class Solution {
    private static Map<Character, String> numberStrs = new HashMap<>();
    static {
        numberStrs.put('2', "abc");
        numberStrs.put('3', "def");
        numberStrs.put('4', "ghi");
        numberStrs.put('5', "jkl");
        numberStrs.put('6', "mno");
        numberStrs.put('7', "pqrs");
        numberStrs.put('8', "tuv");
        numberStrs.put('9', "wxyz");
    }
    public List<String> letterCombinations(String digits) {
        List<String> result = new ArrayList<>();
        if (digits == null || digits.length() == 0) {
            return result;
        }

        realLetterCombinations(digits, "", 0, result);
        return result;
    }

    public void realLetterCombinations(String digits, String prefix, int index, List<String> result) {
        if (index == digits.length()) {
            result.add(prefix);
            return;
        }

        String numberStr = numberStrs.get(digits.charAt(index));
        for (int i = 0; i < numberStr.length(); i++ ) {
            realLetterCombinations(digits, prefix + numberStr.charAt(i), index + 1, result);
        }
    }
}