173_BinarySearchTreeIterator.java

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

public class BSTIterator {
    private Stack<TreeNode> stack = new Stack<>();;
    public BSTIterator(TreeNode root) {
        pushAll(root);        
    }

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

    /** @return the next smallest number */
    public int next() {
        TreeNode temp = stack.pop();
        pushAll(temp.right);
        return temp.val;
    }
    private void pushAll(TreeNode root) {
        while(root != null) {
            stack.push(root);
            root = root.left;
        }
    }
}

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

//
public class BSTIterator {
    
    Stack<TreeNode> stack = new Stack<TreeNode>();
    TreeNode cur;
    public BSTIterator(TreeNode root) {
        cur = root;
    }

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

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

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