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98. Validate Binary Search Tree

中文文档

Description

Given the root of a binary tree, determine if it is a valid binary search tree (BST).

A valid 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.

 

Example 1:

Input: root = [2,1,3]
Output: true

Example 2:

Input: root = [5,1,4,null,null,3,6]
Output: false
Explanation: The root node's value is 5 but its right child's value is 4.

 

Constraints:

  • The number of nodes in the tree is in the range [1, 104].
  • -231 <= Node.val <= 231 - 1

Solutions

Python3

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    pre = None
    def isValidBST(self, root: TreeNode) -> bool:
        if not root:
            return True
        if not self.isValidBST(root.left):
            return False
        if self.pre is not None and self.pre >= root.val:
            return False
        self.pre = root.val
        if not self.isValidBST(root.right):
            return False
        return True

Java

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    private Integer pre = null;
    public boolean isValidBST(TreeNode root) {
        if (root == null) return true;
        if (!isValidBST(root.left)) return false;
        if (pre != null && pre >= root.val) return false;
        pre = root.val;
        if (!isValidBST(root.right)) return false;
        return true;
    }
}

JavaScript

/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {TreeNode} root
 * @return {boolean}
 */
var isValidBST = function (root) {
    let isValidBSTRec = function (root, min, max) {
        if (!root) {
            return true;
        }
        if (root.val <= min || root.val >= max) {
            return false;
        }
        return isValidBSTRec(root.left, min, root.val) && isValidBSTRec(root.right, root.val, max);
    }
    return isValidBSTRec(root, -Infinity, Infinity);
};

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