8/26刷题电话号码的字母组合

1.电话号码的字母组合

思路一：回溯

class Solution {
public:
    vector<string> letterCombinations(string digits) {
        vector<string> combinations;
        if (digits.empty()) {
            return combinations;
        }//边界条件
        unordered_map<char, string> phoneMap{
            {'2', "abc"},
            {'3', "def"},
            {'4', "ghi"},
            {'5', "jkl"},
            {'6', "mno"},
            {'7', "pqrs"},
            {'8', "tuv"},
            {'9', "wxyz"}
        };//哈希表存储每个数字对应的字母
        string combination;
        backtrack(combinations, phoneMap, digits, 0, combination);//字符串数组，哈希表，数字字符串，索引，字母字符串
        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);//索引超过给定电话号码的长度，就生成一个解
        } else {
            char digit = digits[index];
            const string& letters = phoneMap.at(digit);//依次回溯digit里面的每个字母
            for (const char& letter: letters) {
                combination.push_back(letter);
                backtrack(combinations, phoneMap, digits, index + 1, combination);
                combination.pop_back();
            }
        }
    }
};