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.

AC代码

class Solution {
public:
    vector<string> letterCombinations(string digits) {
        if (digits.size() == 0) return {};
        vector<string> v{"abc", "def",  "ghi", "jkl",
                         "mno", "pqrs", "tuv", "wxyz"};
        int size = 1;
        for (int i = 0; i < digits.size(); ++i)
            size *= v[digits[i] - '0' - 2].size();
        vector<string> ans(size);
        int res = size;
        for (int idx = 0; idx < digits.size(); ++idx) {
            int len = v[digits[idx] - '0' - 2].size();
            for (int i = 0; i < size; ++i) {
                ans[i] += v[digits[idx] - '0' - 2][(i / (res / len)) % len];  #核心
            }
            res /= len;
        }
        return ans;
    }
};

总结

网上有使用递归的解法，个人不是很懂递归的思想，有时间学习后再补充解法