Problem 333: Largest BST Subtree
https://leetcode.com/problems/largest-bst-subtree/#/description

思路
通过递归,不断判断更大一些的子树。但这里面有很多易错的地方
isValid()
方程的参数,用 Integer 类型来代表 min 和 max 而不是Integer.MIN_VALUE
,因为后面需要用null
来判断是否为空,要不要继续递归BST 要求每个值不能相同,所以是
>=
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
public class Solution {
public int largestBSTSubtree(TreeNode root) {
if (root == null) return 0;
if (root.left == null && root.right == null) return 1;
if (isValid(root, null, null)) return countNodes(root);
return Math.max(largestBSTSubtree(root.left), largestBSTSubtree(root.right));
}
private boolean isValid(TreeNode root, Integer min, Integer max) {
if (root == null) return true;
if (min != null && root.val <= min) return false;
if (max != null && root.val >= max) return false;
return isValid(root.left, min, root.val) && isValid(root.right, root.val, max);
}
private int countNodes(TreeNode root) {
if (root == null) return 0;
if (root.left == null && root.right == null) return 1;
return 1 + countNodes(root.left) + countNodes(root.right);
}
}
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