Binary Search Tree Iterator

07/16/2016 Tree Stack Design

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 time and uses memory, where h is the height of the tree.

Solution

Result: Accepted Time: 32 ms

Here should be some explanations.

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class BSTIterator {
    vector<TreeNode*> stck;
public:
    BSTIterator(TreeNode *root) {
        while(root)
        {
             stck.push_back(root);
             root = root->left;
        }
    }

    /** @return whether we have a next smallest number */
    bool hasNext() {
        return stck.size() > 0;
    }

    /** @return the next smallest number */
    int next() {
        TreeNode * tmp = stck.back();
        stck.pop_back();
        int ret = tmp->val;
        tmp = tmp->right;
        while(tmp)
        {
            stck.push_back(tmp);
            tmp = tmp->left;
        }
        return ret;
    }
};

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

Complexity Analytics

  • Time Complexity:
  • Space Complexity: