[leetcode]333. Largest BST Subtree最大二叉搜索树子树

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.

题意：

给定二叉树，找出最大的二叉搜索树子树。

思路：

代码：

class Solution {
    public int largestBSTSubtree(TreeNode root) {
        if(root == null) return 0;
        return dfs(root)._size;
    }
    
    private Result dfs(TreeNode root){
        if(root == null) return new Result (true, 0, 999, -999);
        
        Result leftResult = dfs(root.left);
        Result rightResult = dfs(root.right);
        
        boolean isBST = ((root.right == null ||(rightResult._isBST) && (rightResult._min > root.val)) && (root.left == null || (leftResult._isBST)&&(leftResult._max < root.val )));
        int size = isBST ? (leftResult._size + rightResult._size +1) : Math.max(leftResult._size, rightResult._size);
        int min  = root.left == null ? root.val : leftResult._min;
        int max = root.right == null ? root.val : rightResult._max;
        return new Result(isBST, size, min, max);
            
    }
    
    private class Result{
        boolean _isBST;
        int _size;
        int _min;
        int _max;
        private Result(boolean isBST, int size, int min, int max){
            _isBST = isBST;
            _size = size;
            _min = min;
            _max = max;
        }
    }    
}