Given a digit string, return all possible letter combinations that the number could represent.
不知道有多少数字,很难遍历,那就用遍历的极端情况,递归
package pack;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
class Solution {
public List<String> letterCombinations(String digits) {
HashMap<Integer, String> map = new HashMap<Integer, String>();
map.put(2, "abc");
map.put(3, <