<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> 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++;
        }

        vector<unordered_set<int>> groupGraph(m, unordered_set<int>());
        vector<vector<int>> itemGraph(n, vector<int>());
        vector<int> itemIndegree(n, 0);
        vector<int> groupIndegree(m, 0);

        for(int item = 0; item < n; item++){
            for(auto& beforeItem : beforeItems[item]){
                itemGraph[beforeItem].push_back(item);
                int itemGroup = group[item];
                int beforeItemGroup = group[beforeItem];
                if (itemGroup != beforeItemGroup)
                groupGraph[beforeItemGroup].insert(itemGroup);
            }
            itemIndegree[item] += beforeItems[item].size();
        }

        for(int g = 0; g < m; g++){
            for(auto& ng : groupGraph[g]) groupIndegree[ng]++;
        }

        vector<int> groupTopo;
        queue<int> q;
        for(int g = 0; g < m; g++){
            if (groupIndegree[g] == 0) q.push(g);
        }

        while(!q.empty()){
            int g = q.front(); q.pop();
            groupTopo.push_back(g);
            for(auto& ng : groupGraph[g]){
                groupIndegree[ng]--;
                if (groupIndegree[ng] == 0){
                    q.push(ng);
                }
            }
        }

        vector<vector<int>> groupToItems(m, vector<int>());

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

        while(!q.empty()){
            int item = q.front(); q.pop();
            groupToItems[group[item]].push_back(item);
            for(auto& adjItem : itemGraph[item]){
                itemIndegree[adjItem]--;
                if (itemIndegree[adjItem] == 0){
                    q.push(adjItem);
                }
            }
        }

        vector<int> ans;
        for(auto& g : groupTopo){
            for(auto& item : groupToItems[g]) ans.push_back(item);
        }

        if (ans.size() < n) return {};
        return ans;
    }
};
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