[leetcode-17]Letter Combinations of a Phone Number(java)

问题描述：

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”].

分析：这道题比较简单，可能稍微需要动动脑子的地方就是最后如何拼接成字符串的形式。这里使用链表来模拟队列。

代码如下：284ms

[code]public class Solution {
     private String getString(int number){
        switch (number){
            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 null;
        }
    }
    public List<String> letterCombinations(String digits) {
        List<String> res = new LinkedList<String>();
        int length = digits.length();
        String[] strings = new String[length];

         if(length<=0)
            return res;
        for(int i = 0;i<length;i++){
           strings[i] = getString(digits.charAt(i));
        }
        for(int row = 0;row<strings[0].length();row++){
            res.add(new String(new char[]{strings[0].charAt(row)}));
        }
        for(int col = 1;col<length;col++){
            int size = res.size();
            for(int i = 0;i<size;i++){
                String tmp = res.get(0);
                String next = strings[col];
                res.remove(0);
                for(int j = 0;j<next.length();j++)
                    res.add(tmp+next.charAt(j));
            }
        }
        return res;
    }
}