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[leetcode] Validate Binary Search Tree
Given a binary tree, determine if it is a valid binary search tree (BST).
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.
https://oj.leetcode.com/problems/validate-binary-search-tree/
思路1：递归求解，每个节点有一对valid的范围参数。
思路2：中序遍历，看是否是递增序列。
public class Solution { public boolean isValidBST(TreeNode root) { return check(root, Integer.MIN_VALUE, Integer.MAX_VALUE); } private boolean check(TreeNode root, int min, int max) { if (root == null) return true; if (root.val >= max || root.val <= min) return false; return check(root.left, min, root.val) && check(root.right, root.val, max); } public static void main(String[] args) { TreeNode root = new TreeNode(10); root.left = new TreeNode(5); root.left.right = new TreeNode(9); root.right = new TreeNode(20); root.right.left = new TreeNode(11); System.out.println(new Solution().isValidBST(root)); }}
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