Binary Search Tree Iterator

Implement an iterator over a binary search tree (BST). Your iterator will be initialized with the root node of a BST.

Calling next() will return the next smallest number in the BST.

Note: next() and hasNext() should run in average O(1) time and uses O(h) memory, where h is the height of the tree.

Analyse: Inorder traversal of BST. 

Runtime: 28ms.

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class BSTIterator {
public:
    BSTIterator(TreeNode *root) {
        inorder(root);
    }

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

    /** @return the next smallest number */
    int next() {
        int nextSmallest = qu.front();
        qu.pop();
        return nextSmallest;
    }
private:
    queue<int> qu;
    
    void inorder(TreeNode* root) {
        if(!root) return;
        
        inorder(root->left);
        qu.push(root->val);
        inorder(root->right);
    }
};

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