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

Binary tree [2,1,3], return true.

Example 2:

    1
   / 
  2   3

Binary tree [1,2,3], return false.

题目含义：判断一个二叉树是否为二叉搜索树

思路：按照中序将树转换为数组，然后判断是否为递增

    public void inorder(TreeNode root) {
        if (root == null) return;
        inorder(root.left);
        values.add(root.val);
        inorder(root.right);
    }
    
    public boolean isValidBST(TreeNode root) {
        inorder(root);
        for (int i = 0; i < values.size() - 1; i++) {
            if (values.get(i) >= values.get(i + 1)) return false;
        }
        return true;        
    }