[LeetCode] Letter Combinations of a Phone Number

[Problem]

Given a digit string, return all possible letter combinationsthat the number could represent.

A mapping of digit to letters (just like on the telephonebuttons) 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 answercould be in any order you want.

[Analysis]
使用map进行数字到字符串的映射，再递归。

[Solution]

class Solution {
public:
    // combination letters
    vector<string> combination(string digits, map<char, string> &letterTable){
        vector<string> res;
   
        // empty string
        if(digits.length() <= 0){
            res.push_back("");
            return res;
        }
        // one digit string
        else if(digits.length() == 1){
            char ch = digits[0];
            if(ch >= '0' && ch <= '9'){
                string letters = letterTable[ch];
                for(int i = 0; i < letters.length(); ++i){
                    res.push_back(letters.substr(i, 1));
                }
            }
        }
        else{
            char ch = digits[0];
            if(ch >= '0' && ch <= '9'){
                string letters = letterTable[ch];
                vector<string> r = combination(digits.substr(1, digits.length()-1), letterTable);
                for(int i = 0; i < letters.length(); ++i){
                    for(int j = 0; j < r.size(); ++j){
                        res.push_back(letters.substr(i, 1) + r[j]);
                    }
                }
            }
        }
        return res;
    }
   
    // letter combinations
    vector<string> letterCombinations(string digits) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
       
        // letter table
        map<char, string> letterTable;
        letterTable['0'] = " ";
        letterTable['1'] = "";
        letterTable['2'] = "abc";
        letterTable['3'] = "def";
        letterTable['4'] = "ghi";
        letterTable['5'] = "jkl";
        letterTable['6'] = "mno";
        letterTable['7'] = "pqrs";
        letterTable['8'] = "tuv";
        letterTable['9'] = "wxyz";
   
        return combination(digits, letterTable);
    }
};

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