from collections import defaultdict, deque

def sortItems(n, m, group, beforeItems):
    def topological_sort(outdegree, edges):
        res = []
        q = deque([i for i in range(len(outdegree)) if outdegree[i] == 0])
        while q:
            u = q.pop()
            res.append(u)
            for v in edges[u]:
                outdegree[v] -= 1
                if outdegree[v] == 0:
                    q.append(v)
        return res

    # Calculate outdegrees and dependencies for groups and items
    group_outdegree = [0] * m
    group_edges = defaultdict(list)

    item_outdegree = [0] * n
    item_edges = defaultdict(list)

    for i in range(n):
        for dep in beforeItems[i]:
            a, b = group[i], group[dep]
            if a != -1 and a != b and not (group_edges[b] and group_edges[b][-1] == a):
                group_edges[b].append(a)
                group_outdegree[a] += 1
            if a != b:
                item_edges[dep].append(i)
                item_outdegree[i] += 1

    group_order = topological_sort(group_outdegree, group_edges)
    if len(group_order) < m:
        return []

    item_order = topological_sort(item_outdegree, item_edges)
    if len(item_order) < n:
        return []

    # Combine orders
    res = [0] * n
    idx = 0
    for gi in group_order:
        for item_idx in item_order:
            if group[item_idx] == gi:
                res[idx] = item_idx
                idx += 1

    return res

The algorithm consists of the following steps:

1. Calculate the outdegrees and edges for both the groups and the items based on their dependencies.
2. Perform a topological sort on group dependencies and item dependencies separately. If the topological order of groups or items is not complete, return an empty list because we cannot find a valid solution.
3. Combine the orders based on grouping. For each group in the order of groups, find the items belonging to that group from itemOrder and add it to the final result.
4. Return the combined order as the final result.

#include <vector>
#include <algorithm>

std::vector<int> sortItems(int n, int m, std::vector<int>& group, std::vector<std::vector<int>>& beforeItems) {
    // Calculate outdegrees and dependencies for groups and items
    std::vector<int> groupOutdegree(m, 0);
    std::vector<std::vector<int>> groupEdges(m);
    std::vector<int> itemOutdegree(n, 0);
    std::vector<std::vector<int>> itemEdges(n);

    for (int i = 0; i < group.size(); ++i) {
        for (const int dep : beforeItems[i]) {
            int a = group[i], b = group[dep];
            if (a != -1 && a != b && !groupEdges[b].empty() && groupEdges[b].back() == a) {
                groupEdges[b].push_back(a);
                ++groupOutdegree[a];
            }
            if (a != b) {
                itemEdges[dep].push_back(i);
                ++itemOutdegree[i];
            }
        }
    }

    // Topological sort
    auto topologicalSort = [](const std::vector<int>& outdegree, const std::vector<std::vector<int>>& edges) {
        std::vector<int> res, q;
        for (int i = 0; i < outdegree.size(); ++i)
            if (outdegree[i] == 0)
                q.push_back(i);

        while (!q.empty()) {
            int u = q.back();
            q.pop_back();
            res.push_back(u);

            for (const int v : edges[u])
                if (--const_cast<int&>(outdegree[v]) == 0)
                    q.push_back(v);
        }
        return res;
    };

    std::vector<int> groupOrder = topologicalSort(groupOutdegree, groupEdges);
    if (groupOrder.size() < m)
        return {};

    std::vector<int> itemOrder = topologicalSort(itemOutdegree, itemEdges);
    if (itemOrder.size() < n)
        return {};

    // Combine orders
    std::vector<int> res;
    for (const int gi : groupOrder)
        for (const int idx : itemOrder)
            if (group[idx] == gi)
                res.push_back(idx);

    return res;
}

The algorithm consists of the following steps:

1. Calculate the outdegrees and edges for both the groups and the items based on their dependencies.
2. Perform a topological sort on group dependencies and item dependencies separately. If the topological order of groups or items is not complete, return an empty list because we cannot find a valid solution.
3. Combine the orders based on grouping. For each group in the order of groups, find the items belonging to that group from itemOrder and add it to the final result.
4. Return the combined order as the final result.