【Validate Binary Search Tree】cpp

/**
 * 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:
    bool isValidBST(TreeNode* root) {
            if (!root) return true;
            stack<TreeNode *> sta;
            vector<int> traversal;
            TreeNode *p = root;
            while ( !sta.empty() || p )
            {
                if (p){
                    sta.push(p);
                    p = p->left;
                }
                else{
                    p = sta.top();
                    sta.pop();
                    if ( !traversal.empty() && traversal[traversal.size()-1]>=p->val ) return false;
                    traversal.push_back(p->val);
                    p = p->right;
                }
            }
            return true;
    }
};