https://leetcode.com/problems/validate-binary-search-tree/description/
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
Binary tree [2,1,3], return true.
Example 2:
1
/ \
2 3
Binary tree [1,2,3], return false.
note: by the definition, here it mentions:
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.
time: o(n)
space: o(n)
1 public boolean isValidBST(TreeNode root) {
2 if (root == null ) return true;
3 return helper(root, Long.MIN_VALUE, Long.MAX_VALUE);
4 }
5
6 private boolean helper(TreeNode root, long minValue, long maxValue) {
7 if (root == null) return true ;
8 if (root.val >=maxValue || root.val <= minValue){
9 return false ;
10 }
11 return helper(root.left, minValue, root.val) && helper(root.right, root.val, maxValue);
12 }
