17. Letter Combinations of a Phone Number

简单的递归题

import java.util.*;public class Solution {    public List<String> letterCombinations(String digits) {        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";        int len = digits.length();        List<String> result = new ArrayList<>();        dfsStr(result, new String(""), 0, len, map, digits);        return result;    }    public void dfsStr(List<String> result,String tmp, int index, int length, String[] map, String digits) {        if(digits.length()==0) return;        int k = Integer.valueOf(digits.charAt(index)-'0');        String values = map[k];        for(int i = 0; i < values.length(); ++i) {            if(tmp.length() == length-1) {                result.add(new String(tmp + values.charAt(i)));            } else {                dfsStr(result, tmp + values.charAt(i), index + 1,length, map, digits );            }        }    }////    public static void main(String[] args) {//        Solution solution = new Solution();//        List<String> str = solution.letterCombinations("23");//        for(int i = 0; i < str.size(); ++i) {//            System.out.println(str.get(i));//        }//    }}