学步园

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.

思路：

迭代输入的每一种可能即可。可以认为是inorder的递归访问一个多叉树，以题目中给出的输入输出为例：

nil                     “”
2             a         b         c
3          d  e  f   d  e  f   d  e  f 
           |  |  |   |  |  |   |  |  |
           ad ae af  bd be bf  cd ce cf

题解：

const char* key_mapper[]={
    "", // 0
    "", // 1
    "abc",
    "def",
    "ghi",
    "jkl",
    "mno",
    "pqrs",
    "tuv",
    "wxyz"
};

class Solution {
public:
    void generate_it(const string& str, 
        string::const_iterator iter, 
        string& generated, 
        vector<string>& ret_lib)
    {
        if (iter == str.cend())
        {
            // reached the last digit
            ret_lib.push_back(generated);
            return;
        }
        else 
        {
            int digit = *iter - '0';
            const char* ch_list = key_mapper[digit];
         
            // enumerate substitutions   
            for(int i=0; i < strlen(ch_list); ++i)
            {
                generated.push_back(ch_list[i]);
                generate_it(str, next(iter), generated, ret_lib);
                generated.pop_back();
            }
        }
    }
    
    vector<string> letterCombinations(string digits) {
        vector<string> ret;
        string scratch;
        generate_it(digits, digits.cbegin(), scratch, ret);
        return ret;
    }
};