Add Binary Search Tree Iterator

Author: chachaxw

README.md

@@ -31,6 +31,7 @@ collect some good articles about data structure and algorithm.
 10. [排序算法大全](https://zh.wikipedia.org/wiki/Category:排序算法)
 
 ## 常用数据结构(Basic Data Structure)
+
 1. [链表(Linked List)](https://zh.wikipedia.org/wiki/链表)
 2. [二叉树(Binary Tree)](https://zh.wikipedia.org/wiki/二叉树)
 3. [霍夫曼编码(Huffman Coding)](https://zh.wikipedia.org/wiki/霍夫曼编码)

leetcode/binary_search_tree/binary_search_tree_iterator.cpp

@@ -0,0 +1,72 @@
+/**
+ * @Author: Chacha 
+ * @Date: 2018-12-16 21:53:13 
+ * @Last Modified by: Chacha
+ * @Last Modified time: 2018-12-16 23:03:38
+ */
+
+/***********************************************************************************
+ * 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.
+ * 
+ * Example
+ *  For the following binary search tree, in-order traversal by using iterator is [1, 6, 10, 11, 12]
+ * 
+ *         10
+ *       /    \
+ *      1      11
+ *       \       \
+ *       6       12
+ * 
+ * Note: next() and hasNext() should run in average O(1) time and 
+ * uses O(h) memory, where h is the height of the tree.
+ * 
+ * Source: https://leetcode.com/explore/learn/card/introduction-to-data-structure-binary-search-tree/140
+************************************************************************************/
+
+#include <iostream>
+#include <vector>
+#include <stack>
+using namespace std;
+
+/**
+ * Definition for a binary tree node.
+ */
+struct TreeNode {
+    int val;
+    TreeNode *left;
+    TreeNode *right;
+    TreeNode(int x) : val(x), left(NULL), right(NULL) {}
+};
+
+
+class BSTIterator {
+private:
+    stack<TreeNode*> nStack;
+    TreeNode* curt;
+public:
+    BSTIterator(TreeNode *root) {
+        curt = root;
+    }
+
+    /** @return whether we have a next smallest number */
+    bool hasNext() {
+        return (curt != NULL || !nStack.empty());
+    }
+
+    /** @return the next smallest number */
+    int next() {
+        while (curt != NULL) {
+            nStack.push(curt);
+            curt = curt->left;
+        }
+        
+        if (!nStack.empty()) {
+            TreeNode* node = nStack.top();
+            nStack.pop();
+            curt = curt->right;
+        }
+    }
+}