Created
December 10, 2020 05:07
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Binary Search Tree Iterator
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""" | |
Implement the BSTIterator class that represents an iterator over the in-order traversal 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? | |
""" | |
# Definition for a binary tree node. | |
# class TreeNode: | |
# def __init__(self, val=0, left=None, right=None): | |
# self.val = val | |
# self.left = left | |
# self.right = right | |
class BSTIterator: | |
def __init__(self, root: TreeNode): | |
self.sorted = [] | |
self.index = -1 | |
self._inorder(root) | |
def _inorder(self, root: TreeNode): | |
if root: | |
self._inorder(root.left) | |
self.sorted.append(root.val) | |
self._inorder(root.right) | |
def next(self) -> int: | |
self.index += 1 | |
return self.sorted[self.index] | |
def hasNext(self) -> bool: | |
return self.index + 1 < len(self.sorted) | |
# Your BSTIterator object will be instantiated and called as such: | |
# obj = BSTIterator(root) | |
# param_1 = obj.next() | |
# param_2 = obj.hasNext() |
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