C++ 电话号码的字母组合

  给定一个仅包含数字 2-9 的字符串，返回所有它能表示的字母组合。答案可以按 任意顺序 返回。

  给出数字到字母的映射如下（与电话按键相同）。注意 1 不对应任何字母。

输入：digits = "23"
输出：["ad","ae","af","bd","be","bf","cd","ce","cf"]

输入：digits = ""
输出：[]

输入：digits = "2"
输出：["a","b","c"]

提示：

• 0 <= digits.length <= 4
• digits[i] 是范围 ['2', '9'] 的一个数字。

回溯算法，在当前题目非常适合。先使用map容器记录所有可能，在回溯遍历即可。

class Solution {
public:
    void TrackBack(string& digits,int charStart,int size,string& back,vector& res,std::unordered_map& maps)
    {
        if(back.length() == size)
        {
            res.push_back(back);
            return;
        }
        int length = maps[digits[charStart]].length();
        std::string str = maps[digits[charStart]];
        for(int i = 0;i < length; i++)
        {
            back.push_back(str[i]);
            TrackBack(digits,charStart + 1,size,back,res,maps);
            back.pop_back();
        }
    }

    vector letterCombinations(string digits) {
        vector arrs;
        if(digits == "")
            return arrs;
        std::unordered_map maps{{'2',"abc"},{'3',"def"},{'4',"ghi"},
        {'5',"jkl"},{'6',"mno"},{'7',"pqrs"},{'8',"tuv"},{'9',"wxyz"}};
        string back;
        TrackBack(digits,0,digits.length(),back,arrs,maps);
        return arrs;
    }
};

递归法，使用递归求解出所有的可能性并合并结果即可。

class Solution {
public:
    vector CharCombine(string digits,unordered_map& maps)
    {
        vector tmp;
        int length = digits.size();
        //边界条件（当长度为0或1时直接跳出）
        if(1 == length)
        {
            string str = maps[digits[0]];
            for(int j = 0;j < str.length();j++)
            {
                string s = "";
                tmp.push_back(s.append(1,str[j]));
            }
            return tmp;
        }
        else if(0 == length)
            return tmp;
        else//递归
        {
            string str = maps[digits[length-1]];
            vector res = CharCombine(digits.substr(0,length-1),maps);
            for(int i = 0;i < str.length();i++)
            {
                for(int j = 0;j < res.size();j++)
                {
                    string t = res[j];
                    tmp.push_back(t.append(1,str[i]));
                }
            }
            return tmp;
        }
    }

    vector letterCombinations(string digits) {
        std::unordered_map maps{{'2',"abc"},{'3',"def"},{'4',"ghi"},
        {'5',"jkl"},{'6',"mno"},{'7',"pqrs"},{'8',"tuv"},{'9',"wxyz"}};
        return CharCombine(digits,maps);
    }
};