LeetCode ——电话号码的字母组合

题目如下：

采用递归的方式，判断是否到达字符串尾部，到达尾部则添加结果到result。

代码如下：

map<char, string> cs{ { '2', "abc" }, { '3', "def" }, { '4', "ghi" }, { '5', "jkl" }, { '6', "mno" },
{ '7', "pqrs" }, { '8', "tuv" }, { '9', "wxyz" } };
vector<string> result;
void combine(const string &s,string tmp, int index)
{
    string current = cs[s[index]];
    if (index == s.size() - 1)
    {
        for (int i = 0; i < current.size(); i++)
            result.push_back(tmp + current[i]);
        return;
    }
    for (int i = 0; i < current.size(); i++)
        combine(s, tmp+current[i], index + 1);

}
vector<string> letterCombinations(string digits) {
    result.clear();
    int length = digits.size();
    if (length < 1)
        return vector<string>();
    combine(digits, "", 0);
    return result;
}