Day 45

/**
 * 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 {
    private int numOfCameras = 0;
    
    public int minCameraCover(TreeNode root) {
        return dfs(root) == -1 ? numOfCameras + 1 : numOfCameras;
    }
    
    // -1: NOT MONITORED
    //  0: MONITORED
    //  1: HAS CAMERA
    private int dfs(TreeNode root) {
        if (root == null) return 0;
        
        int left = dfs(root.left);
        int right = dfs(root.right);
        
        if (left == -1 || right == -1) {
            numOfCameras++;
            return 1; 
        }
        
        if (left == 1 || right == 1)
            return 0; 
        
        return -1;
    }
}