void topsort(vvi& g, vi& used, vi& ts, int u) {
used[u] = 1;
for(int v : g[u])
if( !used[v] )
topsort(g, used, ts, v);
ts.eb(u);
}
bool topsort(vvi& g, vector<char>& color, vi& ts, int u) {
color[u] = 1;
for(int v : g[u]) {
if( color[v] == 2 )
continue;
if( color[v] == 1 || !topsort(g,color,ts,v) )
return false;
}
ts.eb(u);
color[u] = 2;
return true;
}