LeetCode OJ - Letter Combinations of a Phone Number

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"].

呃，其实看到题目还是有想法的，除了一个一个按顺序找，我还想出按照digit长度的奇偶性分，但是水平有限写不出来哇~~

然后参照：http://blog.csdn.net/china_wanglong/article/details/38495355

用了第一种递推的方法，加了个特殊情况：digits.length()==0

my answer:

import java.util.*;

public class Solution {
public static List<String> letterCombinations(String digits) {
String[] symbol = new String[]{
"","", "abc", "def", "ghi", "jkl", "mno",
"pqrs", "tuv", "wxyz"
};
List<String> ret = new ArrayList<String>();
char[] middleTemp = new char[digits.length()];
if(digits.length()==0){
return ret;
}
dfsGetStr(digits,0,middleTemp,symbol,ret);
return ret;
}

private static void dfsGetStr(String digits,int index,char[]middleStr,String[] symbol,List<String> ret){
if(index == digits.length()) {
ret.add(new String(middleStr));
return ;
}
char strChar = digits.charAt(index);
for(int i=0; i<symbol[strChar-'0'].length(); i++) {
middleStr[index] = symbol[strChar-'0'].charAt(i);
dfsGetStr(digits, index+1, middleStr, symbol, ret);
}
}

public static void main(String[] args){
List<String> ans = letterCombinations("234");
System.out.println(ans);

}
}

Runtime: 40
ms

击败了0.07%

果然效率是低低的。。嗯嗯 继续好好学习 然后把自己的想法写出来