Validate Binary Search Tree, Leetcode 解题笔记

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.

这题其实很简单的递归,但是有个陷阱,要解决这种特殊情况:{10,5,15,#,#,6,20}

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public boolean isValidBST(TreeNode root) {
        return helper(root, Integer.MIN_VALUE, Integer.MAX_VALUE);
    }
    public boolean helper(TreeNode root, int min, int max){
        if(root == null) return true;
        if(root.val <= min || root.val >= max){
            return false;
        }
        return helper(root.left, min, root.val) && helper(root.right, root.val, max);
    }
}
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