[Leetcode] validate Binary Search Tree

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 keysLessThe node's key.
• The right subtree of a node contains only nodes with keysGreaterThe node's key.
• Both the left and right Subtrees must also be binary search trees.

Algorithm ideas:

Train of Thought 1: Find the sequential traversal sequence and determine whether it is actually an incremental sequence. The spatial complexity is O (n)

 

Idea 2: binary classification. The upper and lower boundaries of each vertex are definite.

The Code is as follows:

 1 public class Solution { 2    public boolean isValidBST(TreeNode root) { 3         if(root == null || (root.left == null && root.right == null)) return true; 4         return isValid(root, Integer.MAX_VALUE, Integer.MIN_VALUE); 5     } 6     private boolean isValid(TreeNode root,int biggest,int smallest){ 7         if(root == null ) return true; 8         if(root.val >= biggest || root.val <= smallest) return false; 9         else 10             return isValid(root.left, root.val, smallest) && isValid(root.right, biggest, root.val);11     }12 }