Clean C++ solution

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    class BSTIterator {
        stack<TreeNode*> st;
        void update(TreeNode* cur) {
            for (; cur; cur = cur->left) {
                st.push(cur);                               // push candidate left nodes into stack all the way
        BSTIterator(TreeNode *root) {
            update(root);                                   // add root to stack
        /** @return whether we have a next smallest number */
        bool hasNext() {
            return st.size();                               // has next as long as stack is not empty
        /** @return the next smallest number */
        int next() {
            TreeNode* cur =; st.pop();         
            update(cur->right);                             // get leftmost availabe node and add in its candidate right node
            return cur->val;

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    The problem has the following requirement:
    "Note: next() and hasNext() should run in average O(1) time and uses O(h) memory, where h is the height of the tree."
    It seems that function update(TreeNode * node) has O(h) time complexity (i.e., traverse to leftmost leaf of input node), so I think next() also has O(h) time complexity instead of O(1) since it calls update(TreeNode * node) to find the leftmost leaf of its right subtree.

    Am I missing something?

  • 0

    Suppose we have N as the total number of tree node.

    By calling update() in next(), each tree node will get pushed into the stack exactly once, and get popped out from the stack exactly once.
    Also, next() will get called N times in total. So the average run time is still O(1).

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