LeetCode-in-All
98. Validate Binary Search Tree
Medium
Given the root of a binary tree, determine if it is a valid binary search tree (BST).
A valid 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:
Input: root = [2,1,3]
Output: true
Example 2:
Input: root = [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.
Constraints:
- The number of nodes in the tree is in the range
[1, 104]. -231 <= Node.val <= 231 - 1
Solution
type TreeNode struct {
Val int
Left *TreeNode
Right *TreeNode
}
/**
* Definition for a binary tree node.
* type TreeNode struct {
* Val int
* Left *TreeNode
* Right *TreeNode
* }
*/
func isValidBST(root *TreeNode) bool {
return checkBST(root, nil, nil)
}
func checkBST(root *TreeNode, min, max *int) bool {
if root == nil {
return true
}
if min != nil && *min >= root.Val {
return false
}
if max != nil && root.Val >= *max {
return false
}
return checkBST(root.Left, min, &root.Val) &&
checkBST(root.Right, &root.Val, max)
}