98. 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.

Example 1:

2

/
1 3

Input: [2,1,3]
Output: true
Example 2:

5

/
1 4
/
3 6

Input: [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.

解题思路

var preNode *TreeNode

func isValidBST(root *TreeNode) bool {
	if root == nil {
		return true
	}
	preNode = nil
	return inOrder(root)
}

func inOrder(root *TreeNode) bool {
	if root == nil {
		return true
	}
	if !inOrder(root.Left) {
		return false
	}
	if preNode != nil && root.Val <= preNode.Val {
		return false
	}
	preNode = root
	return inOrder(root.Right)
}