(트리) 두 갈래 트리가 BST인지 여부 판단

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    bool isValidBST(TreeNode *root) {
        return isValidBST(root, INT_MIN, INT_MAX);
    }
    bool isValidBST(TreeNode *root, int low, int high){
        if (root == NULL )
            return true;
        if (low < root->val && root->val < high)
            return (isValidBST(root->left, low, root->val) && isValidBST(root->right, root->val, high));
        else
            return false;
    }
};

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    bool isValidBST(TreeNode *root) {
        vector<int> res;
        isValidBST(root, res);
        int len = res.size();
        bool flag = true;
        for (int i=0; i1; i++){
            if (res[i] >= res[i+1]){
                flag = false;
                break;
            }
        }
        return flag;
    }
    void isValidBST(TreeNode *root, vector<int> &res){
        if (root == NULL)
            return;
        
        isValidBST(root->left, res);
        res.push_back(root->val);
        isValidBST(root->right, res);
    }
};