[LeetCode] 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.

class BSTIterator {
private:
    stack<TreeNode*> st;
public:
    BSTIterator(TreeNode *root) {
        findLeft(root);
    }

    bool hasNext() {
        if (st.empty()) return false;
        return true;
    }

    int next() {
        TreeNode* top = st.top();
        st.pop();
        if (top->right != NULL)
            findLeft(top->right);
        return top->val;
    }

    void findLeft(TreeNode* root)
    {
        TreeNode* p = root;
        while (p != NULL)
        {
            st.push(p);
            p = p->left;
        }
    }
};