17. Letter Combinations of a Phone Number（46.02%）

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题目：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.
这里写图片描述
Input:Digit string “23”
Output: [“ad”, “ae”, “af”, “bd”, “be”, “bf”, “cd”, “ce”, “cf”].
理解：输入数字字符串，输出数字键盘代表的字母的所有组合，一个数字可能代表多个字母，但是这些字母不能同时出现。用递归解决。唯一不太好的地方就是，数字键盘1表示语音拨号，它是没有代表任何一个字母的，这个我直接返回空集合了。

import java.util.ArrayList;import java.util.List;public class Solution {    public List<String> letterCombinations(String digits) {        ArrayList<String> finalStrings = new ArrayList<>();        if (digits == null || digits.equals("")) {            return finalStrings;        }        char[] digitArray = digits.toCharArray();        indexArray(digitArray, 0, "", finalStrings);        return finalStrings;    }    public void indexArray(char[] digitArray, int index, String tempSingle, ArrayList<String> finalArray) {        if (index > digitArray.length-1) {            return;        }        char[] characters = numForCharacters(digitArray[index]);        if (characters != null) {            if (index == digitArray.length-1) {                for (char c : characters) {                    finalArray.add(tempSingle+c);                }            } else {                for (char c : characters) {                    indexArray(digitArray, index+1, tempSingle+c, finalArray);                }            }        }    }    public char[] numForCharacters(char num) {        char[] characters = null;        switch (num) {        case '0':            characters = new char[] {' '};            break;        case '1':            break;        case '2':            characters = new char[] {'a','b','c'};            break;        case '3':            characters = new char[] {'d','e','f'};            break;        case '4':            characters = new char[] {'g','h','i'};            break;        case '5':            characters = new char[] {'j','k','l'};            break;        case '6':            characters = new char[] {'m','n','o'};            break;        case '7':            characters = new char[] {'p','q','r','s'};            break;        case '8':            characters = new char[] {'t','u','v'};            break;        case '9':            characters = new char[] {'w','x','y','z'};            break;        default:            break;        }        return characters;    }}

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