[LeetCode] Largest BST Subtree

Largest BST Subtree

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.

Hint:

You can recursively use algorithm similar to 98. Validate Binary Search Tree at each node of the tree, which will result in O(nlogn) time complexity.

Follow up:

Can you figure out ways to solve it with O(n) time complexity?

Solution

public class Solution {
    public int largestBSTSubtree(TreeNode root) {
        if (root == null) return 0;
        if (isValidBST(root)) return largestBSTSubtree(root.left)+largestBSTSubtree(root.right)+1;
        else return Math.max(largestBSTSubtree(root.left), largestBSTSubtree(root.right));
    }
    public boolean isValidBST(TreeNode root) {
        if (root == null) return true;
        return dfs(root, Long.MIN_VALUE, Long.MAX_VALUE);
    }
    public boolean dfs(TreeNode root, long min, long max) {
        if (root == null) return true;
        if (root.val <= min || root.val >= max) return false;
        return dfs(root.left, min, root.val) && dfs(root.right, root.val, max);
    }
}