diff --git a/src/leecode/tree/No173.java b/src/leecode/tree/No173.java
new file mode 100644
index 0000000..4e94bea
--- /dev/null
+++ b/src/leecode/tree/No173.java
@@ -0,0 +1,56 @@
+package leecode.tree;
+
+import sun.tools.jstack.JStack;
+
+import java.util.Stack;
+
+/**
+ * 二叉搜索树迭代器
+ */
+public class No173 {
+
+    public static void main(String[] args) {
+        Integer[] treeArr = new Integer[]{7,3,15,null,null,9,20};
+        TreeNode root = TreeNode.parseTree(treeArr);
+        BSTIterator iterator = new BSTIterator(root);
+        while (iterator.hasNext()) {
+            System.out.println(iterator.next());
+        }
+    }
+
+    public static class BSTIterator {
+        private Stack<TreeNode> stack = new Stack();
+        private TreeNode now;
+        public BSTIterator(TreeNode root) {
+            while (root != null) {
+                stack.push(root);
+                root = root.left;
+            }
+        }
+        /** @return the next smallest number */
+        public int next() {
+            now = stack.pop();
+            int result = now.val;
+            if (now.right != null){
+                TreeNode right = now.right;
+                if (right.left != null) {
+                    stack.push(right);
+                    while (right.left != null) {
+                        stack.push(right.left);
+                        right = right.left;
+                    }
+                    return result;
+                }
+                stack.push(now.right);
+            }
+            return result;
+        }
+
+        /** @return whether we have a next smallest number */
+        public boolean hasNext() {
+            return (now != null && now.right != null) || !stack.isEmpty();
+        }
+    }
+
+
+}