leetcode 17. Letter Combinations of a Phone Number

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本文介绍了一个简单的算法问题——电话号码的字母组合。给定一个仅包含数字的字符串，返回所有可能的由这些数字对应的字母组成的组合。文章提供了一个C++实现的示例代码，展示了如何通过迭代方式生成所有可能的组合。

Given a digit string, 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.

Input:Digit string "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.

解：就是暴力！

class Solution {
public:
    vector<string> letterCombinations(string digits) {
        vector<string> letter = {"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
        int len1 = digits.size();
        vector<string> res;
        if(len1 == 0) return vector<string>();
        res.push_back("");
        for(int i = 0; i < len1; ++i){
            int num = digits[i] - '0';
            if(num < 0 || num > 9) break;
            vector<string> tmp;
            for(int j = 0; j < letter[num].size(); ++j){
                for(int k = 0; k < res.size(); ++k){
                    tmp.push_back(res[k] + letter[num][j]);
                }
            }
            res.swap(tmp);
        }
        return res;
    }
};