Validate-Binary-Search-Tree.cpp

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