Assume a BST is defined as follows:
The left subtree of a node contains only nodes with keys less than the node's key.
The right subtree of a node contains only nodes with keys greater than the node's key.
Both the left and right subtrees must also be binary search trees.
题目要求判断一颗树是不是二叉搜索数,有关二叉搜索树的性质在这里就不在赘述了。我们采用递归的思想,借助两个变量max,和min,从根节点开始,递归比较每一颗子树,对于左子树的递归我们让max等于子树root的值,对于右子树我们让min等于子树root的值。代码如下:
/** * Definition for a binary tree node. * public class TreeNode { * int val; * TreeNode left; * TreeNode right; * TreeNode(int x) { val = x; } * } */ public class Solution { public boolean isValidBST(TreeNode root) { return isValid(root, null, null); } public 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); } }