[leetcode]Binary Search Tree Iterator

https://leetcode.com/problems/binary-search-tree-iterator/

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.

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

public class BSTIterator {
Stack<TreeNode> stack = null;

public BSTIterator(TreeNode root) {
stack = new Stack<TreeNode>();
while(root != null) {
stack.push(root);
root = root.left;
}
}

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

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

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