leetcode 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.

class Solution {
	TreeNode*pre;
	bool f;
	void leftrootright(TreeNode*node)
	{
		if (node&&node->left)
			leftrootright(node->left);
		if (node)
		{
			if (!pre)
				pre = node;
			else
			{
				if (pre->val >= node->val)
					f = false;
				pre = node;
			}
			cout << node->val;
		}
		if (node&&node->right)
			leftrootright(node->right);
	}
public:
	bool isValidBST(TreeNode* root) {
		if (root == NULL)
			return true;
		pre = NULL;
		f = true;
		leftrootright(root);
		return f;
	}
};

accepted