> For the complete documentation index, see [llms.txt](https://jimmylin1991.gitbook.io/practice-of-algorithm-problems/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://jimmylin1991.gitbook.io/practice-of-algorithm-problems/string/49.-group-anagrams.md).

# 49. Group Anagrams

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.

{% hint style="info" %}
用STL的sort來把每個字排序。
{% endhint %}

```cpp
// Hashtable + STL sort
vector<vector<string>> groupAnagrams(vector<string>& strs) { // time: O(n*str_len*log(str_len)); space: O(n)
    unordered_map<string, vector<string> > m;
    for (string& str : strs) {
        string word = str;
        sort(word.begin(), word.end());
        m[word].push_back(str);
    }
    vector<vector<string> > res;
    for (auto it = m.begin(); it != m.end(); ++it) {
        res.push_back(it->second);
    }
    return res;
}
```

{% hint style="info" %}
也可以自己寫一個簡易的sort來排序string，time complexity比較好。
{% endhint %}

```cpp
// Hashtable + sort
string strSort(string& s) {
    vector<int> count(26, 0);
    int n = s.length();
    for (char c : s) {
        ++count[c - 'a'];
    }
    int pos = 0;
    string res(n, 'a');
    for (int i = 0; i < 26; ++i) {
        for (int j = 0; j < count[i]; ++j) {
            res[pos++] += i;
        }
    }
    return res;
}
vector<vector<string>> groupAnagrams(vector<string>& strs) { // time: O(n * str_len); space: O(n)
    unordered_map<string, vector<string> > group;
    for (string& str : strs) {
        string word = strSort(str);
        group[word].push_back(str);
    }
    vector<vector<string> > res;
    for (auto it = group.begin(); it != group.end(); ++it) {
        res.push_back(it->second);
    }
    return res;
}
```


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