LeetCode 968 Binary Tree Cameras - State Transitions

$state[v] \in \{0, 1, 2\}$: $0$=has camera, $1$=covered (no camera), $2$=not covered (needs parent's camera). If any child is not covered (state $2$), $v$ must have a camera (state $0$).

If any child has camera (state $0$), $v$ is covered (state $1$). Otherwise, all children are covered (state $1$), so $v$ is not covered (state $2$) and relies on its parent. The three states capture all possibilities. The transitions make sure every node gets covered without redundant cameras.