Largest BST Subtree

Java

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    private int[] nullNodeArray = {Integer.MAX_VALUE, Integer.MIN_VALUE, 0};
    
    // [0] - min value
    // [1] - max value
    // [2] - BST count
    public int largestBSTSubtree(TreeNode root) {
        return dfsBSTSubtree(root)[2];
    }
    
    private int[] dfsBSTSubtree(TreeNode root) {
        if (root == null) {
            return nullNodeArray;
        }
        
        int[] left = dfsBSTSubtree(root.left);
        int[] right = dfsBSTSubtree(root.right);
        if (root.val > left[1] && root.val < right[0]) {
            return new int[]{Math.min(root.val, left[0]), Math.max(root.val, right[1]), left[2] + right[2] + 1};
        } 
        return new int[]{Integer.MIN_VALUE, Integer.MAX_VALUE, Math.max(left[2], right[2])};
    }
}