## Tree - 98. Validate Binary Search Tree

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98. Validate Binary Search Tree

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:

``````    2
/ \
1   3

Input: [2,1,3]
Output: true

``````

Example 2:

``````    5
/ \
1   4
/ \
3   6

Input: [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.
``````

go：

``````/**

* Definition for a binary tree node.
* type TreeNode struct {
*     Val int
*     Left *TreeNode
*     Right *TreeNode
* }
*/
func isValidBST(root *TreeNode) bool {
return isValid(root, math.MinInt64, math.MaxInt64)
}

func isValid(node *TreeNode, min, max int) bool {
if node == nil {
return true
}
if node.Val >= max || node.Val <= min {return false}
return isValid(node.Left, min, node.Val) && isValid(node.Right, node.Val, max)
}
``````