<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 {
    vector<int> topologicalSort(vector<vector<int>> &graph, vector<int> indegree){
        vector<int> visited,stack,empty;
        for(int i = 0;i<graph.size();i++){
            if(indegree[i] == 0)stack.push_back(i);
        }

        while(!stack.empty()){
            int cur = stack.back();
            visited.push_back(cur);
            stack.pop_back();
            for (auto &neigh :graph[cur]){
                indegree[neigh] -= 1;
                if(indegree[neigh] == 0)stack.push_back(neigh);
            }
        }

        if(visited.size() == graph.size())return visited;
        else return empty;
    }
public:
    vector<int> sortItems(int n, int m, vector<int>& group, vector<vector<int>>& beforeItems) {
        int gid= m;
        for(int g = 0;g<group.size();g++){
            if(group[g] == -1){
                group[g] = gid;
                gid += 1;
            }
        }

        vector<vector<int>> item_graph(n,vector<int>());
        vector<int> item_indegree(n,0);

        vector<vector<int>> group_graph(gid,vector<int>());
        vector<int> group_indegree(gid,0);

        for(int curr = 0;curr<n;curr++){
            for(auto &prev: beforeItems[curr]){
                item_graph[prev].push_back(curr);
                item_indegree[curr]+=1;

                if(group[curr] != group[prev]){
                    group_graph[group[prev]].push_back(group[curr]);
                    group_indegree[group[curr]] += 1;

                }
            }
        }

        vector<int> item_order = topologicalSort(item_graph,item_indegree);
        vector<int> group_order = topologicalSort(group_graph,group_indegree);

        unordered_map<int,vector<int>> mp;
        for(auto &item: item_order){
            // cout<<item<<endl;
            mp[group[item]].push_back(item);
        }

        vector<int> ans;
        // cout<<"2"<<endl;
        // for(auto &group: group_order){
        //     cout<<group<<endl;
        //     for(auto &v: mp[group])
        //         cout<<v<<" ";
        //     cout<<endl;
        // }
        for(auto &group: group_order)ans.insert(ans.end(),mp[group].begin(),mp[group].end());
        return ans;
    }
};
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