Added BSTIterator implementation

leetcode/design/BSTIterator.java

@@ -0,0 +1,41 @@
+/**
+ * Definition for binary tree
+ * public class TreeNode {
+ *     int val;
+ *     TreeNode left;
+ *     TreeNode right;
+ *     TreeNode(int x) { val = x; }
+ * }
+ */
+
+public class BSTIterator {
+    private static Queue<Integer> queue;
+
+    public BSTIterator(TreeNode root) {
+        queue = new LinkedList<Integer>();
+        inorderTraversal(root);
+    }
+
+    /** @return whether we have a next smallest number */
+    public boolean hasNext() {
+        return !(queue.size()==0);
+    }
+
+    /** @return the next smallest number */
+    public int next() {
+        return queue.poll();
+    }
+
+    private static void inorderTraversal(TreeNode node){
+        if(node==null) return;
+        inorderTraversal(node.left);
+        queue.add(node.val);
+        inorderTraversal(node.right);
+    }
+}
+
+/**
+ * Your BSTIterator will be called like this:
+ * BSTIterator i = new BSTIterator(root);
+ * while (i.hasNext()) v[f()] = i.next();
+ */