[LeetCode] 173. Binary Search Tree Iterator_Medium_tag: Binary Search Tree

mplement an iterator over a binary search tree (BST). Your iterator will be initialized with the root node of a BST.

Calling next() will return the next smallest number in the BST.

 

Example:

BSTIterator iterator = new BSTIterator(root);
iterator.next();    // return 3
iterator.next();    // return 7
iterator.hasNext(); // return true
iterator.next();    // return 9
iterator.hasNext(); // return true
iterator.next();    // return 15
iterator.hasNext(); // return true
iterator.next();    // return 20
iterator.hasNext(); // return false

 

Note:

• next() and hasNext() should run in average O(1) time and uses O(h) memory, where h is the height of the tree.
• You may assume that next() call will always be valid, that is, there will be at least a next smallest number in the BST when next() is called.

 

Because it is Binary Search Tree, so use inorder travesal, so the tree can be transformed in the initialization of stack, and with a pointer to the location where the current node, if more than stack, then there is no next.

 

T: O(n), S: O(n)

Code：

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class BSTIterator(object):
    def inOrder(self, root):
        if not root: return 
        self.inOrder(root.left)
        self.stack.append(root.val)
        self.inOrder(root.right)

    def __init__(self, root):
        """
        :type root: TreeNode
        """
        self.stack = []
        self.point = 0
        self.inOrder(root)
        

    def next(self):
        """
        @return the next smallest number
        :rtype: int
        """
        if self.hasNext():
            nextNum = self.stack[self.point]
            self.point += 1 
            return nextNum
        

    def hasNext(self):
        """
        @return whether we have a next smallest number
        :rtype: bool
        """
        if not self.stack or self.point >= len(self.stack):
            return False
        return True
        


# Your BSTIterator object will be instantiated and called as such:
# obj = BSTIterator(root)
# param_1 = obj.next()
# param_2 = obj.hasNext()