Validate Binary Search Tree LeetCode

Recursion + 利用Binary Search Tree的特点。

• 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.

Java Code：

public boolean isValidBST(TreeNode root) {

        // IMPORTANT: Please reset any member data you declared, as

        // the same Solution instance will be reused for each test case.

        return (root==null)||(isValidBST(root.left) && isValidBST(root.right)

        && checkMax(root.left, root.val)&&checkMin(root.right,root.val));

    }

    

    private boolean checkMax(TreeNode root, int max){

        if(root==null) return true;

        while(root!=null && root.right != null){

            root=root.right;

        }

        return root.val < max;

    }

    

    private boolean checkMin(TreeNode root, int min){

        if(root==null) return true;

        while(root!=null && root.left != null){

            root=root.left;

        }

        return root.val > min;

    }

Updating two new solutions:
First, recursion with null check.
Second, use inorder traversal.