98. Validate Binary Search Tree

98. Validate Binary Search Tree


class Solution {
    public boolean isValidBST(TreeNode root) {
      return helper(root, Long.MIN_VALUE, Long.MAX_VALUE);
    }
    public boolean helper(TreeNode root, long minval, long maxval){
      if(root == null) return true;
      if(root.val >= maxval || root.val <= minval) return false;
      return helper(root.left, minval, root.val) && helper(root.right, root.val, maxval);
        
    }
}

This is kinda similar to preorder recursively . 
First arrive at the root, check the root and go down on the left, check if its valid , and then check right leaf node , using call stack going up a level check th right subtree 

// long: The long data type is a 64-bit signed two's complement integer. It has a minimum value of -9,223,372,036,854,775,808 and a maximum value of 9,223,372,036,854,775,807 (inclusive). Use this data type when you need a range of values wider than those provided by int.

 带返回值的 recursion. time： O(N), N is the number of nodes in the bst, space is O(1) , no extra space is used

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.