leetcode@ [173] Binary Search Tree Iterator (InOrder traversal)

https://leetcode.com/problems/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.

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class BSTIterator {
public: queue<int> Q;
public:
    BSTIterator(TreeNode *root) {
        inOrder(root, Q);
    }
    
    void inOrder(TreeNode* root, queue<int>& Q) {
        if(root != NULL) {
            inOrder(root->left, Q);
            Q.push(root->val);
            inOrder(root->right, Q);
        }
    }

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

    /** @return the next smallest number */
    int next() {
        int next_min = Q.front();
        Q.pop();
        return next_min;
    }
};

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