<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:
    vector<int> topoSort(int sz, vector<vector<int>> &adj){
        vector<int> in(sz);
        vector<int> ans;
        for(int i = 0 ; i< sz; i++){
             for(auto node : adj[i]){
                 in[node]++;
             }
        }

        queue<int> q;
        for(int i = 0; i < sz; i ++){    
            if(in[i] == 0) q.push(i);
        }

        while(!q.empty()){
            int curr = q.front();
            ans.push_back(curr);
            q.pop();
            for(auto child : adj[curr]){
                in[child]--;
                if(in[child]==0){
                    q.push(child);
                }
            }
        }
        return (ans.size()==sz) ? ans : vector<int>{} ;
    }
    
    vector<int> sortItems(int n, int m, vector<int>& group, vector<vector<int>>& beforeItems) {
        for(int i = 0 ; i< n ; i++){
            if(group[i]==-1){
                group[i] = m;
                m++;
            }
        }

        vector<vector<int>> adjItems(n);
        vector<vector<int>> adjGroups(m);

        for(int i=0;i<n;i++){
            vector<int> node = beforeItems[i];  
            for(auto child : node){
                adjItems[child].push_back(i);
                if(group[child]!=group[i]){ 
                        adjGroups[group[child]].push_back(group[i]);
                }
            }
        }

        vector<int> sortedItems = topoSort(n, adjItems);
        vector<int> sortedGroups = topoSort(m, adjGroups);

        vector<vector<int>> sortedItemsGroup(m);
        for(auto item : sortedItems){
            sortedItemsGroup[group[item]].push_back(item);
        }

        vector<int> ans;
        for(auto grp : sortedGroups){
            for(auto node : sortedItemsGroup[grp]){
                ans.push_back(node);
            }
        }

        return ans;
    }
};
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