class Solution {
    final int COVERED = 1000;

    public int minCameraCover(TreeNode root) {
        int cameras = 0;

        for(;;) {
            TreeNode top = null, mid = null, bottom = null, current = root;
            while (current != null) {
                top = mid;
                mid = bottom;
                bottom = current;
                current = (current.left != null) ? current.left: current.right;
            }

            if (!isCovered(bottom)) {
                cameras++;
                setCovered(top);
                setCovered(mid);
                if (mid != null) {
                    setCovered(mid.left);
                    setCovered(mid.right);
                }
            }

            if (mid == null) break;

            cut(mid, bottom);
        }

        return cameras;
    }

    private boolean isCovered(TreeNode node) {
        return node != null && node.val >= COVERED;
    }

    private void setCovered(TreeNode node) {
        if (node != null && !isCovered(node)) node.val += COVERED;
    }

    private void cut(TreeNode from, TreeNode to) {
        if (from.left == to) from.left = null;
        else from.right = null;
    }
}