17. Letter Combinations of a Phone Number**

17. Letter Combinations of a Phone Number**

​​https://leetcode.com/problems/letter-combinations-of-a-phone-number/​​

题目描述

Given a string containing digits from ​​2-9​​ inclusive, return all possible letter combinations that the number could represent.

A mapping of digit to letters (just like on the telephone buttons) is given below. Note that 1 does not map to any letters.

Example:

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

Note:

Although the above answer is in lexicographical order, your answer could be in any order you want.

C++ 实现 1

DFS + Backtracing. 注意对 digits 为空的处理.

class Solution {
private:
    unordered_map<char, vector<string>> phone = {
        {'2', {"a", "b", "c"}},
        {'3', {"d", "e", "f"}},
        {'4', {"g", "h", "i"}},
        {'5', {"j", "k", "l"}},
        {'6', {"m", "n", "o"}},
        {'7', {"p", "q", "r", "s"}},
        {'8', {"t", "u", "v"}},
        {'9', {"w", "x", "y", "z"}}
    };
    void combine(const string &digits, int k, string &cur, vector<string> &res) {
        if (k >= digits.size()) {
            res.push_back(cur);
            return;
        }
        for (auto &s : phone[digits[k]]) {
            cur += s;
            combine(digits, k + 1, cur, res);
            cur.pop_back();
        }
    }
public:
    vector<string> letterCombinations(string digits) {
        if (digits.empty()) return {};
        vector<string> res;
        string cur;
        combine(digits, 0, cur, res);
        return res;
    }
};