<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:
    string ans = "";
    int maxTime = INT_MIN;

    int getTime(string res){
        int time = 0;
        string hrs = res.substr(0,2);
        string min = res.substr(2);
        int h = stoi(hrs);
        int m = stoi(min);
        if(h<=23 && h>=0 && m>=0 && m<=59){
            return h*60 + m;
        }
        else{
            return INT_MIN;
        }
    }

    void helper(vector<int>&arr, string &res, vector<bool>&vis){
        if(res.size() == 4){
            if(getTime(res)>maxTime){
                ans = res;
                maxTime = getTime(res);
            }
            return;
        }

        for(int i=0;i<4;i++){
            if(vis[i]==0){
                res+=to_string(arr[i]);
                vis[i] = 1;
                helper(arr, res, vis);
                vis[i] = 0;
                res.pop_back();
            }
        }
    }
    string largestTimeFromDigits(vector<int>& arr) {
        vector<bool>vis(4,0);
        string res="";
        helper(arr, res, vis);
        if(ans!="")
        ans = ans.substr(0,2) + ":" + ans.substr(2);
        
        return ans;
    }
};
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