https://leetcode.com/problems/validate-binary-search-tree/

Leetcode: 98. Validate Binary Search Tree

/**
 * Definition for a binary tree node.
 * type TreeNode struct {
 *     Val int
 *     Left *TreeNode
 *     Right *TreeNode
 * }
 */
func isValidBST(root *TreeNode) bool {
    if root == nil {
        return true
    }
    var helper func(root *TreeNode, min int, max int) bool
    helper = func(root *TreeNode, min int, max int) bool {
        return root.Val > min && root.Val < max && 
        (root.Left == nil || helper(root.Left, min, root.Val)) &&
        (root.Right == nil || helper(root.Right, root.Val, max))
    }
    return helper(root, math.MinInt64, math.MaxInt64)
}