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# 电话号码的字母组合

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。

image-20231228112127470

示例

输入：digits = "23"

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

class Solution {
    private static final String[] MAPPING = new String[]{"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
    private final List<String> ans = new ArrayList<>();
    private char[] digits, path;
    public List<String> letterCombinations(String digits) {
        int n = digits.length();
        if (n == 0) return List.of();
        this.digits = digits.toCharArray();
        path = new char[n];
        dfs(0);
        return ans;
    }
    private void dfs(int i) {
        if (i == digits.length) {
            ans.add(new String(path));
            return;
        }
        for (char c : MAPPING[digits[i] - '0'].toCharArray()) {
            path[i] = c;
            dfs(i + 1);
        }
    }
}