leetcode49

Given an array of strings, group anagrams together.
Example:
Input: ["eat", "tea", "tan", "ate", "nat", "bat"],
Output:
[
["ate","eat","tea"],
["nat","tan"],
["bat"]
]
Note:
* All inputs will be in lowercase.
* The order of your output does not matter.

Map与序列化。O(n*L), n为字符串个数，L为字符串长度
1.让所有的anagram有一个共同的签名，从而让他们能因为这个共性被map聚集到一起。
2.产生签名的方法：a) 给字符串内的字符排序 b)统计各a-z出现频次并把频次拼接到一起。采用b更好，签名长度稳定在26单位左右。

实现：

class Solution {
    public List<List<String>> groupAnagrams(String[] strs) {
        List<List<String>> ans = new ArrayList<>();
        if (strs == null || strs.length == 0) {
            return ans;
        }
        
        Map<String, List<String>> map = new HashMap<>();
        for (String s : strs) {
            String sig = signature(s);
            map.putIfAbsent(sig, new ArrayList<String>());
            map.get(sig).add(s);
        }

        for (List<String> list : map.values()) {
            ans.add(list);
        }
        return ans;
    }
    
    private String signature(String s) {
        int[] cnt = new int[26];
        for (char c : s.toCharArray()) {
            cnt[c - 'a']++;
        }
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < cnt.length; i++) {
            sb.append(cnt[i]).append('#');
        }
        return sb.toString();
    }
}