Binary Search Tree Iterator

1) Use inorder traversal (non-recursion);

2) Please remember inorder non-recursion traversal;

code

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class BSTIterator {
public:
    vector<TreeNode*> stack;
    TreeNode* p;
    BSTIterator(TreeNode *root) {
        p = root;
    }

    /** @return whether we have a next smallest number */
    bool hasNext() {
        if(p != NULL || !stack.empty())
        {
            return true;
        }
        else
        {
            return false;
        }
    }

    /** @return the next smallest number */
    int next() {
        while(p != NULL)
        {
            stack.push_back(p);
            p = p->left;
        }

        p = stack.back();
        stack.pop_back();
        int ret = p->val;
        p = p->right;
        return ret;
    }
};

/**
 * Your BSTIterator will be called like this:
 * BSTIterator i = BSTIterator(root);
 * while (i.hasNext()) cout << i.next();
 */