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package design;

import java.util.Stack;

/**

* Created by gouthamvidyapradhan on 13/08/2017. Implement an iterator over a binary search tree

* (BST). Your iterator will be initialized with the root node of a BST.

*

* <p>Calling next() will return the next smallest number in the BST.

*

* <p>Note: next() and hasNext() should run in average O(1) time and uses O(h) memory, where h is

* the height of the tree.

*

* <p>Solution: The below solution works in average O(1) time and worst case O(h) time using O(h)

* memory. Use a stack to keep track of min value node.

*/

public class BSTIterator {

private Stack<TreeNode> stack;

public static class TreeNode {

int val;

TreeNode left;

TreeNode right;

TreeNode(int x) {

val = x;

}

}

public static void main(String[] args) throws Exception {

TreeNode root = new TreeNode(10);

root.left = new TreeNode(5);

root.left.left = new TreeNode(4);

root.left.right = new TreeNode(8);

root.left.left.left = new TreeNode(1);

root.left.right.left = new TreeNode(7);

root.right = new TreeNode(12);

root.right.left = new TreeNode(11);

root.right.right = new TreeNode(15);

BSTIterator ite = new BSTIterator(root);

System.out.println("Hasnext: " + ite.hasNext());

System.out.println("next: " + ite.next());

System.out.println("next: " + ite.next());

System.out.println("next: " + ite.next());

System.out.println("next: " + ite.next());

System.out.println("next: " + ite.next());

System.out.println("next: " + ite.next());

System.out.println("next: " + ite.next());

System.out.println("next: " + ite.next());

System.out.println("next: " + ite.next());

System.out.println("Hasnext: " + ite.hasNext());

}

public BSTIterator(TreeNode root) {

stack = new Stack<>();

fillStack(root);

}

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

public boolean hasNext() {

return !stack.isEmpty();

}

/** @return the next smallest number */

public int next() {

if (!stack.isEmpty()) {

TreeNode top = stack.pop();

fillStack(top.right);

return top.val;

}

return -1;

}

/**

* Fill stack with min values

*

* @param node curr node to begin with

*/

private void fillStack(TreeNode node) {

TreeNode ite = node;

while (ite != null) {

stack.push(ite);

ite = ite.left;

}

}

}