98. 验证二叉搜索树

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    typedef long long ll;
    vector<ll>p;
    bool check()
    {
        int flag = 1;
        int len = p.size();
        for (int i = 0; i < len-1; i++)
        {
            if (p[i] >= p[i + 1])
            {
                flag = 0;
                break;
            }
        }
        return flag;
    }
    void Inorder(TreeNode* root)
    {
        if (root->left != NULL) //输出左节点
            Inorder(root->left);
        p.push_back(root->val);
        if (root->right != NULL)
            Inorder(root->right);//输出右节点
    }
    bool isValidBST(TreeNode* root) {
        if (root == NULL)
            return true;
        Inorder(root);
        return check();
    }

};

class Solution {
public:
    typedef long long ll;
    //如果左子树的最大值比根节点大，或者右子树的最小值比根节点小，就不是二叉搜索树
    bool check(TreeNode* root,ll lower,ll upper)
    {
        if (root == NULL)
            return true;
        if (root->val <= lower || root->val >= upper)
            return false;
        return check(root->left, lower, root->val) && check(root->right, root->val, upper);
    }
    bool isValidBST(TreeNode* root) {
        //LONG_MIN和LONG_MAX分别是系统可以提供的最小值和最大值，可以理解为负无穷和正无穷
        return check(root, LONG_MIN, LONG_MAX);
    }
};