17. 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.

public class Solution {  
    private String[] alpha = new String[] {  
            " ",  
            "1", "abc", "def",  
            "ghi", "jkl", "mno",  
            "pqrs", "tuv", "wxyz"  
    };  
    private StringBuilder word;  
  
    private void dfs(List<String> res, String digits, int cur) {  
        if (cur >= digits.length()) {  
            res.add(word.toString());  
        } else {  
            for (int i = 0; i < alpha[digits.charAt(cur) - '0'].length(); ++i) {  
                word.append(alpha[digits.charAt(cur) - '0'].charAt(i));  
                dfs(res, digits, cur + 1);  
                word.deleteCharAt(word.length() - 1);  
            }  
        }  
    }  
  
    public List<String> letterCombinations(String digits) {  
        List<String> ret = new ArrayList<String>();  
        word = new StringBuilder();  
        dfs(ret, digits, 0);  
        return ret;  
    }  
}  

解题思路：穷举所有可能的字符串使用dfs来解决。

class Solution:
    # @return a list of strings, [s1, s2]
    def letterCombinations(self, digits):
        def dfs(num, string, res):
            if num == length:
                res.append(string)
                return
            for letter in dict[digits[num]]:
                    dfs(num+1, string+letter, res)
        
        dict = {'2':['a','b','c'],
                '3':['d','e','f'],
                '4':['g','h','i'],
                '5':['j','k','l'],
                '6':['m','n','o'],
                '7':['p','q','r','s'],
                '8':['t','u','v'],
                '9':['w','x','y','z']
                }
        res = []
        length = len(digits)
        dfs(0, '', res)
        return res

class Solution:  
    # @return a list of strings, [s1, s2]  
    def letterCombinations(self, digits):  
        alpha = [" ", "1", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"]  
        res = []  
        word = []  
        def dfs(cur):  
            if cur >= len(digits):  
                res.append(''.join(word))  
            else:  
                for x in alpha[(int)(digits[cur]) - (int)('0')]:  
                    word.append(x)  
                    dfs(cur + 1)  
                    word.pop()  
        dfs(0)  
        return res