Letter Combinations of a Phone Number

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"].

Note:
Although the above answer is in lexicographical order, your answer could be in any order you want.

 

主要是建立数字字母映射表，然后依次对每个数字所对应的字母进行深搜，代码如下：

class Solution {
public:
    vector<string> letterCombinations(string digits) {
        vector<string> vstr(9); //建立数字映射字母表
        char ch = 'a';
        for(int i=1; i<9; ++i) {
            if( i == 6 || i == 8) {
                int cnt = 4;
                while( cnt-- ) vstr[i].push_back(ch++);
            }
            else {
                int cnt = 3;
                while( cnt-- ) vstr[i].push_back(ch++);
            }
        }
        ans.clear();
        string str;
        dfs(digits, vstr, 0, str);  //深搜
        return ans;
    }
    
    void dfs(string& digits, vector<string>& vstr, int k, string& str) {
        if( k == digits.length() ) {    //如果搜到了末尾，str就是答案之一
            ans.push_back(str);
            return;
        }
        int index = digits[k]-'0'-1;    //取对应数字在映射表中的位置
        for(int i=0; i<vstr[index].size(); ++i) {   //对每个数字所代表字母的进行深搜
            str.push_back(vstr[index][i]);
            dfs(digits, vstr, k+1, str);
            str.pop_back();
        }
    }
    
private:
    vector<string> ans;
};