[LeetCode] 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.

代码如下：
import java.util.Stack;

class TreeNode {
int val;
TreeNode left;
TreeNode right;
TreeNode(int x) { val = x; }
}

public class BSTIterator {
TreeNode root;
Stack<TreeNode> stack;
TreeNode temp;
public BSTIterator(TreeNode root) {
this.root=root;
stack=new Stack<TreeNode>();
temp=root;
}

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

/** @return the next smallest number */
public int next() {
int re=0;
if(hasNext()){
while(temp!=null){
stack.add(temp);
temp=temp.left;
}
if(!stack.isEmpty()){
temp=stack.pop();
re=temp.val;
temp=temp.right;
}
}
return re;
}
}

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