<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:
    map<int, vector<int>> grp_mem;
    vector<vector<int>> sub_ord;

    void topo_sort(int grp_ind, vector<vector<int>>& bfr, vector<vector<int>>& sorted_grp, vector<int>& in_degree) {
        queue<int> q;
        for (auto mem : grp_mem[grp_ind]) {
            if (in_degree[mem] == 0) {
                q.push(mem);
            }
        }
        while (!q.empty()) {
            int now = q.front();
            sorted_grp[grp_ind].push_back(now);
            q.pop();
            for (auto it : sub_ord[now]) {
                in_degree[it]--;
                if (in_degree[it] == 0) q.push(it);
            }
        }
    }

    void topo_sort_grp(int m1, vector<int>& in_degree, vector<int>& ans) {
        queue<int> q;
        for (int i = 0; i < m1; i++) {
            if (in_degree[i] == 0) {
                q.push(i);
            }
        }
        while (!q.empty()) {
            int now = q.front();
            ans.push_back(now);
            q.pop();
            for (auto it : sub_ord[now]) {
                in_degree[it]--;
                if (in_degree[it] == 0) q.push(it);
            }
        }
    }

    vector<int> sortItems(int n, int m, vector<int>& group, vector<vector<int>>& beforeItems) {
        int m1 = m;
        for (int i = 0; i < n; i++) {
            if (group[i] == -1) {
                group[i] = m1;
                m1++;
            }
            grp_mem[group[i]].push_back(i);
        }

        vector<vector<int>> sorted_grp(m1, vector<int>());
        vector<int> in_degree(n, 0);
        sub_ord.assign(n, vector<int>());
        for (int i = 0; i < n; i++) {
            for (auto it : beforeItems[i]) {
                if (group[i] == group[it]) {
                    sub_ord[it].push_back(i);
                    in_degree[i]++;
                }
            }
        }

        for (int i = 0; i < m1; i++) {
            topo_sort(i, beforeItems, sorted_grp, in_degree);
            if (sorted_grp[i].size() != grp_mem[i].size()) {
                return vector<int>();
            }
        }

        sub_ord.clear();
        in_degree.clear();
        sub_ord.assign(m1, vector<int>());
        in_degree.assign(m1, 0);
        vector<int> vis(n, 0);

        for (int i = 0; i < n; i++) {
            for (auto it : beforeItems[i]) {
                if (group[i] != group[it]) {
                    sub_ord[group[it]].push_back(group[i]);
                    in_degree[group[i]]++;
                }
            }
        }

        vector<int> ans;
        topo_sort_grp(m1, in_degree, ans);

        if (ans.size() != m1) {
            return vector<int>();
        }

        vector<int> res;
        for (int i = 0; i < ans.size(); i++) {
            for (auto it : sorted_grp[ans[i]]) {
                res.push_back(it);
            }
        }

        return res;
    }
};

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