<p>There are&nbsp;<code>n</code>&nbsp;items each&nbsp;belonging to zero or one of&nbsp;<code>m</code>&nbsp;groups where <code>group[i]</code>&nbsp;is the group that the <code>i</code>-th item belongs to and it&#39;s equal to <code>-1</code>&nbsp;if the <code>i</code>-th item belongs to no group. The items and the groups are zero indexed. A group can have no item belonging to it.</p>

<p>Return a sorted list of the items such that:</p>

<ul>
	<li>The items that belong to the same group are next to each other in the sorted list.</li>
	<li>There are some&nbsp;relations&nbsp;between these items where&nbsp;<code>beforeItems[i]</code>&nbsp;is a list containing all the items that should come before the&nbsp;<code>i</code>-th item in the sorted array (to the left of the&nbsp;<code>i</code>-th item).</li>
</ul>

<p>Return any solution if there is more than one solution and return an <strong>empty list</strong>&nbsp;if there is no solution.</p>

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

<p><strong><img alt="" src="https://assets.leetcode.com/uploads/2019/09/11/1359_ex1.png" style="width: 191px; height: 181px;" /></strong></p>

<pre>
<strong>Input:</strong> n = 8, m = 2, group = [-1,-1,1,0,0,1,0,-1], beforeItems = [[],[6],[5],[6],[3,6],[],[],[]]
<strong>Output:</strong> [6,3,4,1,5,2,0,7]
</pre>

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

<pre>
<strong>Input:</strong> n = 8, m = 2, group = [-1,-1,1,0,0,1,0,-1], beforeItems = [[],[6],[5],[6],[3],[],[4],[]]
<strong>Output:</strong> []
<strong>Explanation:</strong>&nbsp;This is the same as example 1 except that 4 needs to be before 6 in the sorted list.
</pre>

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

<ul>
	<li><code>1 &lt;= m &lt;= n &lt;= 3 * 10<sup>4</sup></code></li>
	<li><code>group.length == beforeItems.length == n</code></li>
	<li><code>-1 &lt;= group[i] &lt;= m - 1</code></li>
	<li><code>0 &lt;= beforeItems[i].length &lt;= n - 1</code></li>
	<li><code>0 &lt;= beforeItems[i][j] &lt;= n - 1</code></li>
	<li><code>i != beforeItems[i][j]</code></li>
	<li><code>beforeItems[i]&nbsp;</code>does not contain&nbsp;duplicates elements.</li>
</ul>

class Solution {
public:
    void kaln( int node , map< int, map<int,vector<int>> > &gig , vector<int> &ans ){
        map<int,vector<int>> mp =  gig[node];
        map<int,int> indegree;
        for( auto v : mp ){
            for( auto i : v.second ){
                indegree[i]++;
            }
        }
        queue<int> q;
        for( auto v : mp ){
            if( indegree.find(v.first) == indegree.end() ) {
               
                q.push(v.first);
            }
        }
        while(!q.empty()){
            int x = q.front();
            q.pop();
            ans.push_back(x);
            for(auto child : mp[x]){
                indegree[child]--;
                if( indegree[child] == 0 ) q.push(child);
            }
        }
        return;
    }
    vector<int> sortItems(int n, int m, vector<int>& group, vector<vector<int>>& beforeItems) {

        vector<int> grp(n);
        for(int i=0;i<n;i++){
            if( group[i] != -1 ) grp[i] = n + group[i];
            else grp[i] = i;
        }
        
        // relation b/w groups.
        map<int,vector<int>> gg;
        map<int, map<int,vector<int>> > gig;
        for( auto child : grp ){
            gig.insert({child,{}});
        }
        for(int i=0;i<n;i++){
            gig[grp[i]].insert({i,{}});
        }
        for(int i=0;i<n;i++){
            for( auto child : beforeItems[i] ){
                if( grp[i] != grp[child] ){
                    gg[grp[child]].push_back(grp[i]);
                }
                else{
                    gig[grp[child]][child].push_back(i);
                }
            }
        }
        for( auto child : grp ){
            if( gg.find(child) == gg.end() ){
                gg[child] = {};
            }
        }
        // map is ready , gtog bhi and insideg bhi.

        map<int,int> indegree;
        for( auto v : gg ){
            for( auto i : v.second ){
                indegree[i]++;
            }
        }
        queue<int> q;
        for( auto v : gg ){
            if( indegree.find(v.first) == indegree.end() ) {
                q.push(v.first);
            }
        }
        
        vector<int> ans;
        
        while(!q.empty()){
            int x = q.front();
            q.pop();
            // cout<<x<<endl;
            kaln(x,gig,ans);
            for( auto child : gg[x] ){
                
                indegree[child]--;
                if(indegree[child] == 0) q.push(child);
                
            }
        }
        if( ans.size() != n ) return {};
        return ans;
    }
};
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