98 validate binary search tree

98. 验证二叉搜索树

go：

/**

 * Definition for a binary tree node.

 * type TreeNode struct {

 *     Val int

 *     Left *TreeNode

 *     Right *TreeNode

 * }
 */

var prev *TreeNode

func isValidBST(root *TreeNode) bool {
    prev = nil
    return inorder(root)
}

func inorder(root *TreeNode) bool {
    if root == nil {
        return true
    }

    if !inorder(root.Left) {
        return false
    }

    if prev != nil && prev.Val >= root.Val {
        return false
    }

    prev = root

    return inorder(root.Right)
}