BiruLyu
7/27/2017 - 4:54 PM

## 173. Binary Search Tree Iterator(#).java

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

public class BSTIterator {

List<Integer> list;
Iterator<Integer> it;

public BSTIterator(TreeNode root) {
list=new ArrayList<Integer>();
inorder(root);
it=list.iterator();
}

public void inorder(TreeNode root){
if(root==null) return;
inorder(root.left);
inorder(root.right);
}

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

/** @return the next smallest number */
public int next() {
return it.next();
}
}

/**
* Your BSTIterator will be called like this:
* BSTIterator i = new BSTIterator(root);
* while (i.hasNext()) v[f()] = i.next();
*/``````
``````/**
* Definition for binary tree
* public class TreeNode {
*     int val;
*     TreeNode left;
*     TreeNode right;
*     TreeNode(int x) { val = x; }
* }
*/

public class BSTIterator {

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

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

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

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