Binary Tree Cameras

Use postorder DFS because the parent decision depends on child coverage states. Let state 0 mean this node needs a camera, 1 mean it has a camera, and 2 mean it is covered without one. Treat null children as covered. After visiting both children, if either child needs a camera, place one at the current node and return has-camera. Otherwise, if either child has a camera, return covered. If both children are covered but have no camera, return needs-camera so the parent can cover this node. After DFS, add one camera if the root returns needs-camera. This greedy local rule is safe because a camera at a parent covers itself, its parent, and both children, while pushing a required camera down is never more helpful. Test one node, a chain, a full small tree, and root needing coverage. Time is O(n) and recursion space is O(h).