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

[Problem]

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.

[Analysis]

典型的backtracking排列组合的问题，没有tricky的地方。用Iteration或Recursion都可以轻易写出。

[Solution]

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 "";
        }
    }
}