(Java）LeetCode-17. Letter Combinations of a Phone Number__Java

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.

這一題也是蠻簡單滴，無非是幾重迴圈即可搞定，為了麻煩麻煩自己，採用了遞迴的想法，雖然代碼簡單了些，犧牲了空間和時間

public class Solution {    char[][] a = {{},{},{'a','b','c'},{'d','e','f'},{'g','h','i'},{'j','k','l'},{'m','n','o'},{'p','q','r','s'},{'t','u','v'},{'w','x','y','z'}};public List<String> letterCombinations(String digits) {        if(digits.length() == 0){        return new ArrayList<String>();        }        if(digits.length() == 1){        List<String> result = new ArrayList<String>();        int i = Integer.parseInt(digits);        for(char c : a[i]){        result.add(Character.toString(c));        }        return result;        }        int j = Integer.parseInt(digits.substring(0,1));        List<String> res = new ArrayList<String>();        for(char c : a[j]){        for(String ss : letterCombinations(digits.substring(1))){        res.add(c+ss);        }        }return res;    }}

附上大神的遞迴，速度很快~

public List<String> letterCombinations(String digits) {    List<String> list = new ArrayList<String>();    if(!digits.isEmpty()) {        helper(digits, "", 0, list);    }    return list;}public void helper(String digits, String combo, int position, List<String> list) {    if(position == digits.length()) {        list.add(combo);        return;    }    String[] letters = getMapping(digits.charAt(position));    for(int i = 0; i < letters.length; i++){        helper(digits, combo + letters[i], position + 1, list);    }}public String[] getMapping(char c) {    switch (c) {    case '2':        return new String[] {"a", "b", "c"};    case '3':        return new String[] {"d", "e", "f"};    case '4':        return new String[] {"g", "h", "i"};    case '5':        return new String[] {"j", "k", "l"};    case '6':        return new String[] {"m", "n", "o"};    case '7':        return new String[] {"p", "q", "r", "s"};    case '8':        return new String[] {"t", "u", "v"};    case '9':        return new String[] {"w", "x", "y", "z"};    }    return new String[] {};}