17. 电话号码的字母组合/C++

回溯法

class Solution {
public:
    vector<string> letterCombinations(string digits) {
        res.clear();
        if(digits.empty())
            return res;
        
        func(digits,0,"");
        return res;
    }
    
private:
	//表示数字对应的字符
    vector<string> table{" ","","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};
    vector<string> res;
    
    //digits代表输入的数字串
    //index表示当前处理第几位
    //str代表index之前位数的字符串
    void func(const string &digits, int index, const string &str){
    	//所有位数都处理完了，将字符串压入数组
        if(digits.size() == index){
            res.push_back(str);
            return;
        }
        
        //获得数字串中的数字
        char c=digits[index];
        //获得对应的字符串
        string letters = table[c-'0'];
        //将index位上可能的字符循环接上str[0...index-1]
        for(int i=0;i<letters.size();++i)
            func(digits,index+1,str+letters[i]);
        return;
    }
};