Grokking 207-b

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {

  int COVER_WITH_CAMERA = 0;
  int COVER_NO_CAMERA = 1;
  int NOT_COVER = 2;

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

    int status = getStatus(root);

    // if root's status is not cover, add a camera to root node
    if (status == NOT_COVER) {
      numCameras++;
    }

    return numCameras;
  }

  public int getStatus(TreeNode node) {
    if (node == null) {
      return COVER_NO_CAMERA;
    }

    int left = getStatus(node.left);
    int right = getStatus(node.right);

    if (left == NOT_COVER || right == NOT_COVER) {
      // if left or right is not covered, add a camera at current node
      numCameras++;
      return COVER_WITH_CAMERA;
    }

    if (left == COVER_NO_CAMERA && right == COVER_NO_CAMERA) {
      return NOT_COVER;
    }

    return COVER_NO_CAMERA;
  }
}