98. Validate Binary Search Tree 检验二叉树是不是bst

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.

    

复习时还不会的地方：不是在主函数里分左右，而是在helper中分左右。因为合理范围的界限需要在递归内部讨论，左子树边是min-root，右子树是root-max

            
                public
                class
                 Solution {
    
                public
                boolean
                 isValidBST(TreeNode root) {
        
                return
                 isValidBST(root, Long.MIN_VALUE, Long.MAX_VALUE);
    }
    
    
                public
                boolean isValidBST(TreeNode root, long minVal, long maxVal) {
        if (root == null) returntrue;
        if (root.val >= maxVal || root.val <= minVal) returnfalse;
        return isValidBST(root.left, minVal, root.val) && isValidBST(root.right, root.val, maxVal);
    }
}
        

原文：https://www.cnblogs.com/immiao0319/p/14997942.html