968. Binary Tree Cameras

/**
 * 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 {
    // 0: it cannot be monitored
    // 1: put a camera here
    // 2: no camera, but can be monitored (a null)
    int total = 0;
    public int minCameraCover(TreeNode root) {
        if (helper(root) == 0) {
            total += 1;
        }
        return total;
    }
    
    private int helper(TreeNode root) {
        if (root == null) return 2;
        int left = helper(root.left);
        int right = helper(root.right);
        
        if (left == 0 || right == 0) {
            total += 1;
            return 1;
        }
        
        if (left == 2 && right == 2) {
            return 0;
        } else return 2;
    }
}