98. 验证二叉搜索树

/**
 * 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:
    bool isValidBST(TreeNode* root) {
        vector<int> v1;
        stack<TreeNode *>s1;

        TreeNode *p = root;
        while(p || !s1.empty()){
            if(p){
                s1.push(p);
                p=p->left;
            }else{
                p = s1.top();
                s1.pop();
                v1.push_back(p->val);
                p=p->right;
            }
        }

        for(int i=0;i<v1.size()-1;i++){
            if(v1[i]>=v1[i+1]){
                return false;
            }
        }

        return true;
    }
};