[333. Largest BST Subtree] #leetcode

333_0903.java

public class Solution {
    
    class Result {  // (size, rangeLower, rangeUpper) -- size of current tree, range of current tree [rangeLower, rangeUpper]
        int size;
        int lower;
        int upper;
        
        Result(int size, int lower, int upper) {
            this.size = size;
            this.lower = lower;
            this.upper = upper;
        }
    }
    
    int max = 0;
    
    public int largestBSTSubtree(TreeNode root) {
        if (root == null) { return 0; }    
        traverse(root);
        return max;
    }
    
    private Result traverse(TreeNode root) {
        if (root == null) { return new Result(0, Integer.MAX_VALUE, Integer.MIN_VALUE); }
        Result left = traverse(root.left);
        Result right = traverse(root.right);
        if (left.size == -1 || right.size == -1 || root.val <= left.upper || root.val >= right.lower) {
            return new Result(-1, 0, 0);
        }
        int size = left.size + 1 + right.size;
        max = Math.max(size, max);
        return new Result(size, Math.min(left.lower, root.val), Math.max(right.upper, root.val));
    }
}