LeetCode - Largest BST Subtree

LargestBSTSubtree.java

// Given a binary tree, find the largest subtree which is a Binary Search Tree (BST), where largest means subtree with largest number of nodes in it.

// Note:
// A subtree must include all of its descendants.
// Here's an example:
//     10
//     / \
//   5  15
//   / \   \ 
// 1   8   7
// The Largest BST Subtree in this case is the highlighted one. 
// The return value is the subtree's size, which is 3.

int max = 0;
public int largestBSTSubtree(TreeNode root) {
    helper(root);
    return max;
}

private int[] helper(TreeNode root){
    int[] res = new int[]{0, Integer.MAX_VALUE, Integer.MIN_VALUE};
    if(root == null) return res;
    int[] left = helper(root.left);
    int[] right = helper(root.right);
    
    if(left[0] == -1 || right[0] == -1 || root.val <= left[2] || root.val >= right[1]){
        res[0] = -1; res[1] = 0; res[2] = 0;
        return res;
    }
    res[0] = left[0] + 1 + right[0];
    max = Math.max(res[0], max);
    res[1] = Math.min(left[1], root.val);
    res[2] = Math.max(right[2], root.val);
    return res;
}