Leetcode--17. Letter Combinations of a Phone Number

题目：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”].

思路如下：

• 把数字表示的字符映射到数组中，初始化result为{“”}。
• 对每一个digit[i]中的数字字符:
  
  • 取result的队首元素，分别与digit[i]中的每个字符连接，放入队列，并删除该队首元素。

注意要先判断digits是否为空，然后再初始化result，测试用例第一个就是digits为空。

C++代码如下：

vector<string> letterCombinations(string digits) {    vector<string> result;    if (digits.empty())        return result;    result.push_back("");    vector<string> letter = { "","","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz" };    for (int i = 0; i < digits.size(); i++)    {        int sz = result.size(); //一定先记录下当前result的size，否则会陷入死循环，因为下面的操作会改变result的size.        for (int j = 0; j < sz; j++)        {            string temp = result[0];            result.erase(result.begin());            for (int k = 0; k < letter[digits[i] - '0'].size(); k++)            {                result.push_back(temp + letter[digits[i] - '0'][k]);            }        }    }    return result;}