Package g0101_0200.s0173_binary_search_tree_iterator

Class BSTIterator

java.lang.Object

g0101_0200.s0173_binary_search_tree_iterator.BSTIterator

public class BSTIterator
extends Object

173 - Binary Search Tree Iterator.<p>Medium</p> <p>Implement the <code>BSTIterator</code> class that represents an iterator over the <strong><a href="https://en.wikipedia.org/wiki/Tree_traversal#In-order_(LNR)" target="_top">in-order traversal</a></strong> of a binary search tree (BST):</p> <ul> <li><code>BSTIterator(TreeNode root)</code> Initializes an object of the <code>BSTIterator</code> class. The <code>root</code> of the BST is given as part of the constructor. The pointer should be initialized to a non-existent number smaller than any element in the BST.</li> <li><code>boolean hasNext()</code> Returns <code>true</code> if there exists a number in the traversal to the right of the pointer, otherwise returns <code>false</code>.</li> <li><code>int next()</code> Moves the pointer to the right, then returns the number at the pointer.</li> </ul> <p>Notice that by initializing the pointer to a non-existent smallest number, the first call to <code>next()</code> will return the smallest element in the BST.</p> <p>You may assume that <code>next()</code> calls will always be valid. That is, there will be at least a next number in the in-order traversal when <code>next()</code> is called.</p> <p><strong>Example 1:</strong></p> <p><img src="https://assets.leetcode.com/uploads/2018/12/25/bst-tree.png" alt="" /></p> <p><strong>Input</strong> [&ldquo;BSTIterator&rdquo;, &ldquo;next&rdquo;, &ldquo;next&rdquo;, &ldquo;hasNext&rdquo;, &ldquo;next&rdquo;, &ldquo;hasNext&rdquo;, &ldquo;next&rdquo;, &ldquo;hasNext&rdquo;, &ldquo;next&rdquo;, &ldquo;hasNext&rdquo;] [<a href="7,-3,-15,-null,-null,-9,-20">7, 3, 15, null, null, 9, 20</a>, [], [], [], [], [], [], [], [], []]</p> <p><strong>Output:</strong> [null, 3, 7, true, 9, true, 15, true, 20, false]</p> <p><strong>Explanation:</strong></p> <pre><code> BSTIterator bSTIterator = new BSTIterator([7, 3, 15, null, null, 9, 20]); bSTIterator.next(); // return 3 bSTIterator.next(); // return 7 bSTIterator.hasNext(); // return True bSTIterator.next(); // return 9 bSTIterator.hasNext(); // return True bSTIterator.next(); // return 15 bSTIterator.hasNext(); // return True bSTIterator.next(); // return 20 bSTIterator.hasNext(); // return False </code></pre> <p><strong>Constraints:</strong></p> <ul> <li>The number of nodes in the tree is in the range <code>[1, 10⁵]</code>.</li> <li><code>0 <= Node.val <= 10⁶</code></li> <li>At most <code>10⁵</code> calls will be made to <code>hasNext</code>, and <code>next</code>.</li> </ul> <p><strong>Follow up:</strong></p> <ul> <li>Could you implement <code>next()</code> and <code>hasNext()</code> to run in average <code>O(1)</code> time and use <code>O(h)</code> memory, where <code>h</code> is the height of the tree?</li> </ul>

Constructor Summary

Constructors

Constructor	Description
BSTIterator(TreeNode root)

Method Summary

Modifier and Type	Method	Description
boolean	hasNext()
int	next()

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Details

BSTIterator

public BSTIterator(TreeNode root)

Method Details

next

public int next()

hasNext

public boolean hasNext()