[Leetcode]Letter Combinations of a Phone Numbers随记

public class Solution {
    public List<String> letterCombinations(String digits) {
        List<String> result;
        String[] map = new String[10];  
        map[0] = "";  
        map[1] = "";  
        map[2] = "abc";  
        map[3] = "def";  
        map[4] = "ghi";  
        map[5] = "jkl";  
        map[6] = "mno";  
        map[7] = "pqrs";  
        map[8] = "tuv";  
        map[9] = "wxyz"; 
        result=dfs(digits,map);
        return result;
    }
    public List<String> dfs(String digits,String[] map){
        List<String> result=new ArrayList<String>();
        if(digits.isEmpty()){
            return result;
        }
        int number=digits.charAt(0)-‘0‘;
        String c=map[number];
        List<String> next=dfs(digits.substring(1),map);
        for(int i=0;i<c.length();i++){
            if(next.isEmpty()){
                result.add(c.charAt(i)+"");
            }
            for(int j=0;j<next.size();j++){
                result.add(c.charAt(i)+next.get(j));
            }
        }
        return result;
    }
}