Thursday, August 20, 2015

Leetcode: Binary Search Tree Iterator

http://buttercola.blogspot.com/2014/12/facebook-binary-tree-iterator.html

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

public class BSTIterator {
    private TreeNode p;
    private Stack<TreeNode> stack = new Stack<TreeNode>();

    public BSTIterator(TreeNode root) {
        this.p = root;
    }

    /** @return whether we have a next smallest number */
    public boolean hasNext() {
        return !stack.isEmpty() || p != null;
    }

    /** @return the next smallest number */
    public int next() {
        while (p != null) {
            stack.push(p);
            p = p.left;
        }
        
        TreeNode curr = stack.pop();
        p = curr.right;
        
        return curr.val;
    }
}

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

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