LEETCODE: 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.

二叉树的性质，各种遍历方法。好像中序遍历是先遍历左子树，在根节点，最后右节点。根几点大于左子树所有节点，并且最大的在左子树中最靠右的。要记住这个位置！

/**
* Definition for binary tree
* struct TreeNode {
* int val;
* TreeNode *left;
* TreeNode *right;
* TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
class Solution {
private:
TreeNode *lastOfLeft;
bool checkValid(TreeNode *root) {
if(root == NULL) return true;
bool isLeftOK = checkValid(root->left);
if(lastOfLeft != NULL && root->val <= lastOfLeft->val) {
return false;
}
lastOfLeft = root;
bool isRightOK = checkValid(root->right);
return isLeftOK && isRightOK;
}
public:
bool isValidBST(TreeNode *root) {
if(root == NULL) return true;
lastOfLeft = NULL;
return checkValid(root);

}
};