98 Validate Binary Search Tree

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本文介绍了一种检查给定二叉树是否为有效二叉搜索树的方法。通过递归方式确认每个节点的值是否在其合法范围内，并确保左右子树也符合二叉搜索树的定义。

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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.

Example 1:

Input:
    2
   / \
  1   3
Output: true

Example 2:

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

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public boolean isValidBST(TreeNode root) {
        return isValidBST(root, Long.MIN_VALUE, Long.MAX_VALUE);
    }
    
    private boolean isValidBST(TreeNode root, long lower, long upper) {
        if (root == null)
            return true;
        return root.val > lower && root.val < upper &&
            isValidBST(root.left, lower, root.val) && 
            isValidBST(root.right, root.val, upper);
    }
}