LeetCode – Largest BST Subtree (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.

Java Solution

class Wrapper{
    int size;
    int lower, upper;
    boolean isBST;
    public Wrapper(){
        lower = Integer.MAX_VALUE;
        upper = Integer.MIN_VALUE;
        isBST = false;
        size = 0;
public class Solution {
    public int largestBSTSubtree(TreeNode root) {
        return helper(root).size;
    public Wrapper helper(TreeNode node){
        Wrapper curr = new Wrapper();
        if(node == null){
            curr.isBST= true;
            return curr;
        Wrapper l = helper(node.left);
        Wrapper r = helper(node.right);
        //current subtree's boundaries
        curr.lower = Math.min(node.val, l.lower);
        curr.upper = Math.max(node.val, r.upper);
        //check left and right subtrees are BST or not
        //check left's upper again current's value and right's lower against current's value
        if(l.isBST && r.isBST && l.upper<=node.val && r.lower>=node.val){
            curr.size = l.size+r.size+1;
            curr.isBST = true;
            curr.size = Math.max(l.size, r.size);
            curr.isBST  = false;
        return curr;
Category >> Algorithms  
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