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.

Medium

Implement the BSTIterator class that represents an iterator over the **[in-order traversal](https://en.wikipedia.org/wiki/Tree_traversal#In-order_(LNR))** of a binary search tree (BST):

• BSTIterator(TreeNode root) Initializes an object of the BSTIterator class. The root 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.
• boolean hasNext() Returns true if there exists a number in the traversal to the right of the pointer, otherwise returns false.
• int next() Moves the pointer to the right, then returns the number at the pointer.

Notice that by initializing the pointer to a non-existent smallest number, the first call to next() will return the smallest element in the BST.

You may assume that next() calls will always be valid. That is, there will be at least a next number in the in-order traversal when next() is called.

Example 1:

Input [“BSTIterator”, “next”, “next”, “hasNext”, “next”, “hasNext”, “next”, “hasNext”, “next”, “hasNext”] [[[7, 3, 15, null, null, 9, 20]], [], [], [], [], [], [], [], [], []]

Output: [null, 3, 7, true, 9, true, 15, true, 20, false]

Explanation:

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 

Constraints:

• The number of nodes in the tree is in the range [1, 105].
• 0 <= Node.val <= 106
• At most 105 calls will be made to hasNext, and next.

Follow up:

• Could you implement next() and hasNext() to run in average O(1) time and use O(h) memory, where h is the height of the tree?

Constructor Summary

Constructors

Constructor and Description
BSTIterator(TreeNode root)

Method Summary

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

Methods inherited from class java.lang.Object

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

Constructor Detail

BSTIterator

public BSTIterator(TreeNode root)

Method Detail

next

public int next()

hasNext

public boolean hasNext()