电话号码的字母组合.md

题目

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"]

提示：

• 0 <= digits.length <= 4
• digits[i] 是范围 ['2', '9'] 的一个数字。

参考答案

class Solution {
public:
    vector<string> letterCombinations(string digits) {
        vector<string> combinations;
        if (digits.empty()) {
            return combinations;
        }
        unordered_map<char, string> phoneMap{
            {'2', "abc"},
            {'3', "def"},
            {'4', "ghi"},
            {'5', "jkl"},
            {'6', "mno"},
            {'7', "pqrs"},
            {'8', "tuv"},
            {'9', "wxyz"}
        };
        string combination;
        backtrack(combinations, phoneMap, digits, 0, combination);
        return combinations;
    }

    void backtrack(vector<string>& combinations, const unordered_map<char, string>& phoneMap, const string& digits, int index, string& combination) {
        if (index == digits.length()) {
            combinations.push_back(combination);
        } else {
            char digit = digits[index];
            const string& letters = phoneMap.at(digit);
            for (const char& letter: letters) {
                combination.push_back(letter);
                backtrack(combinations, phoneMap, digits, index + 1, combination);
                combination.pop_back();
            }
        }
    }
};