173. Binary Search Tree Iterator

[huimeich]

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.

/**
 * Your BSTIterator will be called like this:
 * BSTIterator i = new BSTIterator(root);
 * while (i.hasNext()) v[f()] = i.next();
 */

[huimeich]

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */

public class BSTIterator {

    private List<TreeNode> list;

    public BSTIterator(TreeNode root) {
        this.list = new ArrayList<TreeNode>();
        if (root != null) list.add(root);
        while(root != null && root.left != null){
            list.add(root.left);
            root = root.left;
        }
    }

    /** @return whether we have a next smallest number */
    public boolean hasNext() {
        return !list.isEmpty();
    }

    /** @return the next smallest number */
    public int next() {
        TreeNode root = list.remove(list.size() - 1);
        int val = root.val;
        if (root.right != null) {
            list.add(root.right);
            root = root.right;
            while(root.left != null) {
                list.add(root.left);
                root = root.left;
            }
        }
        return val;
    }
}