LeetCode #17 Letter Combinations of a Phone Number (M)

import java.util.ArrayList;
import java.util.List;

public class Solution {
    public List<String> letterCombinations(String digits) {
        if (digits == null) {
            return null;
        }
        
        List<String> result = new ArrayList<String>();
        StringBuilder buf = new StringBuilder();                
        getLetterCombinations(digits, 0, buf, result);
        
        return result;
    }
    
    public void getLetterCombinations(String digits, int idx, StringBuilder buf, List<String> result) {
        if (idx >= digits.length()) {
            result.add(buf.toString());
        } else {            
            String mappings = getMapping(digits.charAt(idx));
            for (int i = 0; i < mappings.length(); i++) {            
                StringBuilder newBuf = new StringBuilder(buf);
                getLetterCombinations(digits, idx + 1, newBuf.append(mappings.charAt(i)), result);            
            }
        }                
    }
    
    public String getMapping(char digit) {
        switch (digit) {        
            case ‘2‘: return "abc";
            case ‘3‘: return "def";
            case ‘4‘: return "ghi";
            case ‘5‘: return "jkl";
            case ‘6‘: return "mno";
            case ‘7‘: return "pqrs";
            case ‘8‘: return "tuv";
            case ‘9‘: return "wxyz";
            default: return "";
        }
    }
}