<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(unordered_map<int,vector<int>>adj,vector<int>& indegree)
{
queue<int>que;
for(int i=0;i<adj.size();i++)
{
    if(indegree[i]==0)
    {
        que.push(i);
    }
}
    vector<int>result;
    while(!que.empty())
    {
        int u=que.front();
        que.pop();
        result.push_back(u);
        for(int v:adj[u])
        {
            indegree[v]--;
            if(indegree[v]==0)
            {
                que.push(v);
            }
        }
    }
    return result.size()==adj.size()?result: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++;
            }
        }
        unordered_map<int,vector<int>>itemGraph;
        vector<int>itemIndegree(n,0);
        for(int i=0;i<n;i++)
        {
            itemGraph[i]=vector<int>();
        }
        unordered_map<int,vector<int>>groupGraph;
        vector<int>groupIndegree(m,0);
        for(int i=0;i<m;i++)
        {
            groupGraph[i]=vector<int>();
        }

        for(int i=0;i<n;i++)
        {
            for(int prev:beforeItems[i])
            {
                itemGraph[prev].push_back(i);
                itemIndegree[i]++;
                if(group[prev]!=group[i])
                {
                    int prevGroup=group[prev];
                    int currGroup=group[i];
                    groupGraph[prevGroup].push_back(currGroup);
                    groupIndegree[currGroup]++;
                }
            }
        }
        vector<int>itemOrder=topoSort(itemGraph,itemIndegree);
        vector<int>GraphOrder=topoSort(groupGraph,groupIndegree);
        unordered_map<int,vector<int>>groupsToItemOrder;
        for(int& item:itemOrder)
        {
            int itemGroup=group[item];
            groupsToItemOrder[itemGroup].push_back(item);
        }
        vector<int>result;
        for(int& group:GraphOrder)
        {
            result.insert(result.end(),groupsToItemOrder[group].begin(),groupsToItemOrder[group].end());
        }
        return result;
    }
};
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