地址：https://leetcode.com/problems/validate-binary-search-tree/

題目：

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:
  

Example 2:

理解：

判斷一個樹是不是二叉排序樹。

實現1：

採用中序遍歷的方式，得到的應該是一個有序序列，如果其中存在無序元素，就把標記修改，最後返回標記。

class Solution {
	TreeNode* pre = nullptr;
	bool valid = true;
public:
	bool isValidBST(TreeNode* root)
 
 {
		isValid(root);
		return valid;
	}

	void isValid(TreeNode* root) {
		if (!root) return;
		isValid(root->left);
		if (pre&&pre->val >= root->val) valid = false;
		pre = root;
		isValid(root->right);
	}
};

實現2：

看了下別人的實現，使用從上到下的方式，標記這個結點上面的最大值和最小值，一旦不滿足就返回false，這樣比較快。

class
 
 Solution {
public:
	bool isBalanced(TreeNode* curr, long minVal, long maxVal) {
		if (!curr) return true;
		long currVal = static_cast<long>(curr->val);
		if (currVal > maxVal || currVal < minVal) return false;
		return (isBalanced(curr->left, minVal, currVal - 1) &&
			isBalanced(curr->right, currVal + 1, maxVal));
	}


	bool isValidBST(TreeNode* root) {
		return isBalanced(root, LONG_MIN, LONG_MAX);
	}
};