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Leetcode[173] Binary Search Tree Iterator

15 Nov 2015

###Task1 Implement 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.

Note: next() and hasNext() should run in average O(1) time and uses O(h) memory, where h is the height of the tree. ###Python

# 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 __init__(self, root):
        """
        :type root: TreeNode
        """
        self.stack = []
        self.prev = root
        

    def hasNext(self):
        """
        :rtype: bool
        """
        return len(self.stack) > 0 or self.prev

    def next(self):
        """
        :rtype: int
        """
        while len(self.stack) > 0 or self.prev:
            while self.prev:
                self.stack.append(self.prev)
                self.prev = self.prev.left
            cur = self.stack.pop()
            self.prev = cur.right
            return cur.val

# Your BSTIterator will be called like this:
# i, v = BSTIterator(root), []
# while i.hasNext(): v.append(i.next())

###Java

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */

public class BSTIterator {
    Stack<TreeNode> stack = new Stack<TreeNode>();
    TreeNode cur;

    public BSTIterator(TreeNode root) {
        cur = root;
    }

    /** @return whether we have a next smallest number */
    public boolean hasNext() {
        return (cur != null || !stack.isEmpty());
    }

    /** @return the next smallest number */
    public int next() {
        while (cur != null) {
            stack.push(cur);
            cur = cur.left;
        }
        cur = stack.pop();
        TreeNode res = cur;
        cur = cur.right;
        
        return res.val;
    }
}

/**
 * Your BSTIterator will be called like this:
 * BSTIterator i = new BSTIterator(root);
 * while (i.hasNext()) v[f()] = i.next();
 */

###Points

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