0098-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 inOrder(TreeNode* root, vector<int>& ans) {
        if(root == NULL) {
            return;
        }
        
        inOrder(root->left, ans);
        ans.push_back (root->val);
        inOrder(root->right, ans);
    }
    
    bool isValidBST(TreeNode* root) {
        vector<int> ans;
        inOrder(root, ans);
        
        for(int i=0; i<ans.size()-1; i++) {
            if(ans[i] >= ans[i+1]) {
                return false;
            }
        }
        
        return true;
    }
};