public void ComputeSubTreeSize(int root)
{
const long X = 1000000000;
var stack = new FastStack <long>(N + 1);
stack.Push(root * X);
par[root] = -1;
d[root] = 0;
while (stack.Any())
{
var val = stack.Peek();
var u = (int)(val / X);
var it = (int)(val % X);
if (it == G[u].Count)
{
stack.Pop();
subTreeSize[u]++;
if (par[u] >= 0)
{
subTreeSize[par[u]] += subTreeSize[u];
}
}
else
{
var to = G[u][it].to;
stack.Last++;
if (to == par[u])
{
continue;
}
par[to] = u;
d[to] = d[u] + 1;
stack.Push(to * X);
}
}
}
