这道题如果使用DFS比较简单，让人尴尬的是使用迭代反而实现有点小难度，可能平时刷题时侧重于DFS BFS，而较少练习迭代法的原因。

class Solution {
public:
    vector<string> letterCombinations(string digits) {
        if(digits.empty()) return {};
        vector<string> res = {""};
        
        for(auto digit:digits)
        {
            vector<string> temp;
            for(auto candidate:pad[digit-'0'])
            {
                for(auto s:res)
                    temp.push_back(s+candidate);
            }
            //swap执行效率较高，赋值复杂度为O(n)，swap复杂度为O(1)
            //res = temp;
            swap(res,temp);
        }
        
        return res;
    }
private:
    const vector<string> pad = {
        "", "", "abc", "def", "ghi", "jkl",
        "mno", "pqrs", "tuv", "wxyz"
    };
};

执行效率:

Runtime: 0 ms, faster than 100.00% of C++ online submissions for Letter Combinations of a Phone Number.
Memory Usage: 6.5 MB, less than 62.18% of C++ online submissions for Letter Combinations of a Phone Number.

BFS实现方式：

class Solution {
public:
    vector<string> letterCombinations(string digits) {
        if(digits.empty()) return {};
        const vector<string> words{ " ", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz" };
        deque<string> dq = {""};
        for(auto digit:digits)
        {
            int size = dq.size();
            for(int i = 0;i < size;i++)
            {
                string str = dq.front();
                dq.pop_front();
                for(auto c:words[digit-'0'])
                {
                    dq.push_back(str+c);
                }
                    
            }
        }
        
        return vector<string>(dq.begin(),dq.end());
        
    }
    
};

运行结果:

Runtime: 0 ms, faster than 100.00% of C++ online submissions for Letter Combinations of a Phone Number.
Memory Usage: 6.4 MB, less than 84.43% of C++ online submissions for Letter Combinations of a Phone Number.

上面的代码和BFS的套路代码非常像，也相对比较好理解。最后看下DFS的实现:

class Solution {
private:
    int depth;
    const vector<string> board={"","","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};
public:
    vector<string> letterCombinations(string digits) {
        if(digits.empty()) return {};
        vector<string> res;
        string ans;
        depth = digits.size();
        dfs(0,digits,ans,res);
        
        return res;
    }
private:
    void dfs(int cur_dep,const string& digits,string ans,vector<string>& res)
    {
        if(depth == cur_dep)
        {
            res.push_back(ans);
            return;
        }
        int num=digits[cur_dep]-'0';
        for(int i=0;i<board[num].size();i++){
            ans.push_back(board[num][i]);
            dfs(cur_dep+1,digits,ans,res);
            ans.pop_back();
        }
        
    }
};

运行效率：

Runtime: 0 ms, faster than 100.00% of C++ online submissions for Letter Combinations of a Phone Number.
Memory Usage: 6.6 MB, less than 62.18% of C++ online submissions for Letter Combinations of a Phone Number.