Binary Search Tree Iterator

QUESTION

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.

1ST TRY

前序遍历，记录下左子树的遍历路径

/**

 * Definition for binary tree

 * struct TreeNode {

 *     int val;

 *     TreeNode *left;

 *     TreeNode *right;

 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}

 * };

 */



struct MyListNode {

    TreeNode* node;

    MyListNode *next;

    MyListNode(TreeNode* x) : node(x), next(NULL) {}

};

class BSTIterator {

public:

    BSTIterator(TreeNode *root) {

        currentRoot = root;

        leftTree = NULL;

        buildLeftTree(root);

    }



    /** @return whether we have a next smallest number */

    bool hasNext() {

        if(leftTree) return true;

        else return false;

    }



    /** @return the next smallest number */

    int next() {

            ret = leftTree->node->val;

            tmpListNode = leftTree;

            leftTree = leftTree->next;

            buildLeftTree(tmpListNode->node->right);

            delete[] tmpListNode;

            return ret;

    

    }

    

    void buildLeftTree(TreeNode *root)

    {

        if(!root) return;

        tmpTreeNode = root;

        while(tmpTreeNode)

        {

            MyListNode *newListNode = new MyListNode(tmpTreeNode);

            newListNode->next = leftTree;

            leftTree = newListNode;

            tmpTreeNode = tmpTreeNode->left;

        }

    }

private:

    TreeNode *tmpTreeNode;

    TreeNode *currentRoot;

    MyListNode *tmpListNode;

    MyListNode *leftTree;

    int ret;

    

};



/**

 * Your BSTIterator will be called like this:

 * BSTIterator i = BSTIterator(root);

 * while (i.hasNext()) cout << i.next();

 */

Result: Accepted