LeetCode 968 Binary Tree Cameras - Implementation

Post-order DFS. Return the state of each node. Count cameras (state $0$ nodes). Handle root specially: if root returns "not covered", we must add a camera at root. Null children: treat as "covered" (state $1$) so they don't force unnecessary cameras. Time: $O(n)$, Space: $O(h)$. Each node decides based on children's states in constant time. Start from leaves and work up. Leaves are never optimal camera locations because their parents cover more.

Time complexity: $O(n)$.

Space complexity: $O(h)$.