BST Iterator

• Time : O(n)
• Space : O(h)

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
 
 vector<TreeNode*> st;

BSTIterator::BSTIterator(TreeNode *root) {
    st.clear();
    while(root != NULL) {
        st.push_back(root);
        TreeNode* left = root->left;
        root->left = NULL;
        root = left;
    }
}

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

/** @return the next smallest number */
int BSTIterator::next() {
    int res = st.back()->val;
    TreeNode* back = st.back();
    st.pop_back();
    if(back->right != NULL) {
        st.push_back(back->right);
    }
    if(!st.empty()) {
        back = st.back();
        while(back->left != NULL) {
            st.push_back(back->left);
            back->left = NULL;
            back = st.back();
        }
    }
    return res;
}

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