<p>Given an array <code>arr</code> of 4 digits, find the latest 24-hour time that can be made using each digit <strong>exactly once</strong>.</p>

<p>24-hour times are formatted as <code>&quot;HH:MM&quot;</code>, where <code>HH</code> is between <code>00</code> and <code>23</code>, and <code>MM</code> is between <code>00</code> and <code>59</code>. The earliest 24-hour time is <code>00:00</code>, and the latest is <code>23:59</code>.</p>

<p>Return <em>the latest 24-hour time in <code>&quot;HH:MM&quot;</code> format</em>. If no valid time can be made, return an empty string.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> arr = [1,2,3,4]
<strong>Output:</strong> &quot;23:41&quot;
<strong>Explanation:</strong> The valid 24-hour times are &quot;12:34&quot;, &quot;12:43&quot;, &quot;13:24&quot;, &quot;13:42&quot;, &quot;14:23&quot;, &quot;14:32&quot;, &quot;21:34&quot;, &quot;21:43&quot;, &quot;23:14&quot;, and &quot;23:41&quot;. Of these times, &quot;23:41&quot; is the latest.
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> arr = [5,5,5,5]
<strong>Output:</strong> &quot;&quot;
<strong>Explanation:</strong> There are no valid 24-hour times as &quot;55:55&quot; is not valid.
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>arr.length == 4</code></li>
	<li><code>0 &lt;= arr[i] &lt;= 9</code></li>
</ul>

class Solution {
public:
    bool check(string &s)
    {
        int x=stoi(s.substr(0,2));
        if(x>=24)return false;
        int y=stoi(s.substr(2,2));
        if(y>=60)return false;
        return true;
    }
    string ans="";int mx=-1;
    void f(string &s,int i)
    {
        if(s.length()==4)
        {
            if(check(s)){
                if(mx<(stoi(s.substr(0,2))*60+ stoi(s.substr(2,2))))
                {
                    ans=s;
                    mx=(stoi(s.substr(0,2))*60+ stoi(s.substr(2,2)));
                }
            }
        }
        unordered_set<char> s1;
        for(int idx=i;idx<s.size();idx++)
        {
            if(s1.find(s[idx])!=s1.end())continue;
            s1.insert(s[idx]);
            swap(s[i],s[idx]);
            f(s,i+1);
            swap(s[i],s[idx]);
        }
    }
    string largestTimeFromDigits(vector<int>& arr) {
        string s="";
        for(int i=0;i<4;i++)s+=to_string(arr[i]);
        f(s,0);
        if(ans=="")return "";
        ans.insert(ans.begin()+2,':');
        return ans;
    }
};
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