8-1 樹形問題 Letter Combinations of a Phone Number

題目: LeetCode 17. 電話號碼的字母組合

給定一個僅包含數字 2-9 的字串，返回所有它能表示的字母組合。

給出數字到字母的對映如下（與電話按鍵相同）。注意 1 不對應任何字母。

示例:

輸入：“23”
輸出：[“ad”, “ae”, “af”, “bd”, “be”, “bf”, “cd”, “ce”, “cf”].
說明:
儘管上面的答案是按字典序排列的，但是你可以任意選擇答案輸出的順序。

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

/// 17. Letter Combinations of a Phone Number
/// https://leetcode.com/problems/letter-combinations-of-a-phone-number/description/
/// 時間複雜度: O(2^len(s))
/// 空間複雜度: O(len(s))
class Solution {

    private String letterMap[] = {
                " ",    //0
                "",     //1
                "abc"
 
,  //2
                "def",  //3
                "ghi",  //4
                "jkl",  //5
                "mno",  //6
                "pqrs", //7
                "tuv",  //8
                "wxyz"  //9
    };

    private ArrayList<String> res;

    public List<String> letterCombinations(String digits)
 
 {

        res = new ArrayList<String>();
        if(digits.equals(""))
            return res;

        findCombination(digits, 0, "");
        return res;
    }

    // s中儲存了此時從digits[0...index-1]翻譯得到的一個字母字串
    // 尋找和digits[index]匹配的字母, 獲得digits[0...index]翻譯得到的解
    private void findCombination(String digits, int index, String s){

        System.out.println(index + " : " + s);
        if(index == digits.length()){
            res.add(s);
            System.out.println("get " + s + " , return");
            return;
        }

        Character c = digits.charAt(index);
        assert  c.compareTo('0') >= 0 &&
                c.compareTo('9') <= 0 &&
                c.compareTo('1') != 0;
        String letters = letterMap[c - '0'];
        for(int i = 0 ; i < letters.length() ; i ++){
            System.out.println("digits[" + index + "] = " + c +
                    " , use " + letters.charAt(i));
            findCombination(digits, index+1, s + letters.charAt(i));
        }

        System.out.println("digits[" + index + "] = " + c + " complete, return");

        return;
    }

    private static void printList(List<String> list){
        for(String s: list)
            System.out.println(s);
    }

    public static void main(String[] args) {

        printList((new Solution()).letterCombinations("234"));
    }
}