topological sort for directed graph

topsort.cpp

using std::vector;
using std::queue;

vector<int> topsort(const vector<vector<int> > &g) {
    int n = g.size();
    vector<int> d(n);
    for(int i = 0; i < n; i++)
        for(int j = 0; j < g[i].size(); j++)
            d[g[i][j]]++;
    
    queue<int> q;
    vector<int> seq;
    for(int i = 0; i < n; i++)
        if(!d[i]) q.push(i);
    while(!q.empty()) {
        int hd = q.front();
        q.pop();
        seq.push_back(hd);
        for(int i = 0; i < g[hd].size(); i++) {
            int dst = g[hd][i];
            d[dst]--;
            if(!d[dst]) q.push(dst);
        }
    }
    return seq;
}