<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(vector<vector<int>> &edges, int n){
        vector<vector<int>> graph(n);
        vector<int> indegree(n);

        for (auto edge : edges){
            graph[edge[0]].push_back(edge[1]);
            indegree[edge[1]]++;
        }

        queue<int> q; 

        for (int i = 0; i < n; i++){
            if (indegree[i] == 0) q.push(i);
        }

        vector<int> ans;

        while (!q.empty()){
            int node = q.front(); 
            q.pop(); 

            ans.push_back(node);
            for (int child : graph[node]){
                indegree[child]--; 
                if (indegree[child] == 0) q.push(child);
            }
        }

        return ans;
    }

    vector<int> topoSort(vector<vector<int>> &edges, vector<int> &nodes){
        unordered_map<int, vector<int>> graph; 
        unordered_map<int, int> indegree;
        vector<int> ans;

        for (int i = 0; i < nodes.size(); i++){
            graph[nodes[i]] = {};
            indegree[nodes[i]] = 0;
        }

        for (auto edge : edges){
            graph[edge[0]].push_back(edge[1]);
            indegree[edge[1]]++;
        }

        queue<int> q;
        for (auto p : indegree){
            if (p.second == 0) q.push(p.first);
        }

        while (!q.empty()){
            int node = q.front(); 
            q.pop(); 

            ans.push_back(node);

            for (int child : graph[node]){
                indegree[child]--; 
                if (indegree[child] == 0) q.push(child);
            }
        }

        return ans;
    }

    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++;
        }

        // sort groups
        vector<vector<int>> groupEdges; 
        vector<vector<vector<int>>> edges(m);
        vector<vector<int>> items(m);

        for (int i = 0; i < n; i++){
            items[group[i]].push_back(i);
            for (int j = 0; j < beforeItems[i].size(); j++){
                if (group[i] == group[beforeItems[i][j]]) edges[group[i]].push_back({beforeItems[i][j], i});
                else groupEdges.push_back({group[beforeItems[i][j]], group[i]});
            }
        }

        vector<int> groupSorted = topoSort(groupEdges, m);

        if (groupSorted.size() != m) return {};

        vector<vector<int>> withinGroups(m);

        for (int i = 0; i < m; i++){
            withinGroups[i] = topoSort(edges[i], items[i]);
            if (withinGroups[i].size() != items[i].size()) return {};
        }

        vector<int> ans;

        for (int i = 0; i < m; i++){
            ans.insert(ans.end(), withinGroups[groupSorted[i]].begin(), withinGroups[groupSorted[i]].end());
        }

        return ans;
    }
};
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