binary search tree iterator

Author: always-maap

173.BinarySearchTreeIterator.py

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+"""
+173. Binary Search Tree Iterator
+Medium
+Stack | Tree | Design | Binary Search Tree | Binary Tree | Iterator
+---
+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?
+"""
+
+from typing import Optional
+
+
+# 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: Optional[TreeNode]):
+        self.stack = []
+        while root:
+            self.stack.append(root)
+            root = root.left
+
+    def next(self) -> int:
+        popped = self.stack.pop()
+        curr = popped.right
+        while curr:
+            self.stack.append(curr)
+            curr = curr.left
+        return popped.val
+
+    def hasNext(self) -> bool:
+        return self.stack != []
+
+
+# Your BSTIterator object will be instantiated and called as such:
+# obj = BSTIterator(root)
+# param_1 = obj.next()
+# param_2 = obj.hasNext()