Entry-Only Variant

Store only tin[v] for each node. This gives you a DFS preorder traversal numbering.

Use this when you need to check ancestor relationships quickly: $u$ is an ancestor of $v$ if tin[u] < tin[v] and $u$ is on the path from root to $v$. You lose the ability to identify subtree ranges, so this variant is less useful for most problems.

However, combined with tout, you can check ancestry in $O(1)$: $u$ is an ancestor of $v$ if tin[u] <= tin[v] and tout[v] <= tout[u].