Chef and Subtree MEXs

The MEX of a set of integers is defined as the smallest non-negative integer that does not belong to this set. For example, $\mathrm{MEX}(\{0,2,3\}) = 1$ and $\mathrm{MEX}(\{1,3\}) = 0$. Chef has a tree with $N$ nodes (numbered $1$ through $N$). The tree is rooted at node $1$. Chef wants to assign a non-negative integer to each node in such a way that each integer between $0$ and $N-1$ (inclusi