Go: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.
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/**
 * Definition for a binary tree node.
 * type TreeNode struct {
 *     Val int
 *     Left *TreeNode
 *     Right *TreeNode
 * }
 */

var pre *TreeNode

func isValidBST(root *TreeNode) bool {
    sign := true

    pre = nil
    convertToBST(root, &sign)
    return sign
}

func convertToBST(root *TreeNode, sign *bool) {
    if root != nil && *sign == true {
        convertToBST(root.Left, sign)

        if pre != nil && pre.Val >= root.Val {
            *sign = false
        }

        pre = root

        convertToBST(root.Right, sign)
    }
}