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

本文参考了博文：https://blog.csdn.net/ljiabin/article/details/41699241

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.

• type TreeNode struct {

• Val int
  

• Left *TreeNode
  

• Right *TreeNode
  

• }
  */
  func isValidBST(root *TreeNode) bool {

  INT_MAX := int(^uint(0) >> 1)
  INT_MIN := ^INT_MAX

  if root == nil {
  return true
  }

  return validate(root, INT_MAX, INT_MIN)

}

func validate(root *TreeNode, max, min int) bool {
if (root == nil) {
return true
}

if root.Val <= min {
    return false
}


if root.Val >= max {
    return false
}


return validate(root.Left, root.Val, min) && validate(root.Right, max, root.Val)

}