Group Anagrams

Given an array of strings, group anagrams together.
For example, given: ["eat", "tea", "tan", "ate", "nat", "bat"],
Return:[ ["ate", "eat","tea"], ["nat","tan"], ["bat"] ]
Note:

• For the return value, each inner list's elements must follow the lexicographic order.
  
• All inputs will be in lower-case.

题目大意：给定一个字符串数组，将其分类，含有相同数量字母的为一类。要求每一类按字母序返回，所有输入为小写

题目难度：Medium

import java.util.*;
/**
 * Created by gzdaijie on 16/5/23
 * 不同的质数相乘，结果一定不同，可以用不同质数代替字母
 * 判断乘积，即可以判断是否属于同一组，借助hashMap记录键值对
 */
public class Solution {
    public static List<List<String>> groupAnagrams(String[] strs) {
        int[] prime = {2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97, 101, 103};
        List<List<String>> result = new ArrayList<>();
        HashMap<Integer, Integer> hash = new HashMap<>();

        for (String s : strs) {
            int key = 1;
            for (char ch: s.toCharArray()) {
                key *= prime[ch - 'a'];
            }

            if (hash.containsKey(key)) {
                result.get(hash.get(key)).add(s);
            } else {
                List<String> t = new ArrayList<>();
                t.add(s);
                result.add(t);
                hash.put(key, result.size() - 1);
            }
        }

        for (List list: result) {
            Collections.sort(list);
        }
        return result;
    }
}