Leetcode Binary Search Tree Iterator

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class BSTIterator {
private:
   stack<TreeNode *> stk;
   void keep_pushing_left(TreeNode *&root){
       while(root){
            stk.push(root);
            root = root -> left;
        }
   }
public:
    BSTIterator(TreeNode *root) {
        keep_pushing_left(root);
    }

    /** @return whether we have a next smallest number */
    bool hasNext() {
        return !stk.empty();
    }

    /** @return the next smallest number */
    int next() {
        TreeNode *top = stk.top();
        int val = top -> val;
        stk.pop();
        if(top->right){
            keep_pushing_left(top = top->right);
        }
        return val;
    }
};

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