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。

  • 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。

Solution

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#include <iostream>
#include <vector>
#include <map>
using namespace std;

class Solution {
public:
    vector<string> letterCombinations(string digits) {
        static map<int, string> letterC = {{2, "abc"}, {3, "def"}, {4, "ghi"},
                                                {5, "jkl"}, {6, "mno"}, {7, "pqrs"}, {8, "tuv"},
                                                {9, "wxyz"}};
        vector<string> result;
        if (digits.size() == 0) {
            return result;
        }
        result.resize(1);

        for (int i = 0; i < digits.length(); ++i) {
            string letters = letterC[digits[i] - '0'];
            int len = result.size();
            for (int j = 0; j < len; ++j) {
                string str = result[0];
                for (int k = 0; k < letters.length(); ++k) {
                    string temp = str + letters[k];
                    result.push_back(temp);
                }
                result.erase(result.begin());
            }
        }
        return result;
    }
};

int main() {
    vector<string> result;
    result = Solution().letterCombinations("23"); // ad ae af bd be bf cd ce cf
    for (int i = 0; i < result.size(); ++i) {
        std::cout << result[i] << " ";
    }
    std::cout << std::endl;
}