2023-10-31 15:59:17 +0000 UTC

/**
 * Definition for a binary tree node.
 * type TreeNode struct {
 *     Val int
 *     Left *TreeNode
 *     Right *TreeNode
 * }
 */
func isValidBST(root *TreeNode) bool {
    if root == nil {
        return false
    }
    return dfs(root.Left, &root.Val, nil) && dfs(root.Right, nil, &root.Val)
}

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