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.

confused what "{1,#,2,3}" means? > read more on how binary tree is serialized on OJ.

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用中序遍历依次判断当前节点数值是否比前一个节点大就可以了，注意第一个节点数值判断条件

non-recursion版本

bool isValidBST(TreeNode* root) {
    if (root == nullptr)
        return true;
    stack<TreeNode*> sta;
    sta.push(root);
    TreeNode* lastNode = nullptr;
    TreeNode* lastRoot = root;
    while (!sta.empty())
    {
        root = sta.top();
        if (lastRoot != root->right)
        {
            if (lastRoot != root->left) {
                if (root->left != nullptr) {
                    sta.push(root->left);
                    continue;
                }
            }
            if (!lastNode || root->val > lastNode->val)
                lastNode = root;
            else
                return false;
            if (root->right != nullptr) {
                sta.push(root->right);
                continue;
            }
        }
        lastRoot = root;
        sta.pop();
    }
    return true;
}