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"].

Solution

回溯法递归

Complexity

时间复杂度 O(n!),空间复杂度 O(n)

Code

public class Solution {
    public List<String> letterCombinations(String digits) {
        ArrayList<String> res = new ArrayList<String>();
        if (digits.equals(""))
            return res;
        
        String[] keyboard = new String[]{" ","","abc", "def", "ghi", 
            "jkl", "mno", "pqrs", "tuv", "wxyz"};
        letterCombinationsRe(keyboard,res,digits,"");
        return res;
    }
    
    public void letterCombinationsRe(String[] keyboard, 
            ArrayList<String> res, String digits, String s) {
        if (s.length() == digits.length()) {
            res.add(s);
            return;
        }
        String letters = keyboard[digits.charAt(s.length()) - '0'];
        for (int i = 0; i < letters.length(); ++i) {
            letterCombinationsRe(keyboard, res, digits, s+letters.charAt(i));
        }
    }
}