CSES 1674 Subordinates - Core Idea

The number of subordinates of node $v$ is the size of the subtree rooted at $v$, minus $1$ (do not count $v$ itself). Define ext{size}[v] = number of nodes in subtree of $v$ (including $v$ itself). Then subordinates[v] = ext{size}[v] - 1.

This converts the problem into a standard subtree aggregation: compute sizes, then subtract one. This is a common pattern. When the problem asks for something slightly different from what you compute, adjust at the end.