LeetCode 17. Letter Combinations of a Phone Number

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.

这道题其实用递归写更好一些，但是考虑到递归可能会引起栈溢出，我还是用循环来做，方法如下：最外层循环是遍历输入的数字序列，每次读入一个数字，并找到与之相对应的字符串，然后遍历该字符串，同时遍历字符向量，逐个将字符与字符向量中的字符串拼接并且放入临时变量t，每次读完一个数字将t赋值给res，直到读完所有的数字，这道题的时间复杂度为指数复杂度。

class Solution {
public:
    vector<string> letterCombinations(string digits) {
        if (digits.size() == 0)
            return {};
        vector<string> res{""};
        string dict[] = {"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
        for (int i = 0; i < digits.size(); i++)
        {
            vector<string> t;
            string str = dict[digits[i] - '0'];
            for (int j = 0; j < str.size(); j++)
            {
                for (auto s : res)
                    t.push_back(s + str[j]);
            }
            res = t;
        }
        return res;
    }
};