"Binary Search Tree Iterator"

Author: lennylxx

README.md

@@ -127,7 +127,7 @@ The `☢` means that you need to have a LeetCode Premium Subscription.
 | 186 | [Reverse Words in a String II] ☢                             |                    |
 | 179 | [Largest Number]                                             | [C](src/179.c)     |
 | 174 | [Dungeon Game]                                               |                    |
-| 173 | [Binary Search Tree Iterator]                                |                    |
+| 173 | [Binary Search Tree Iterator]                                | [C++](src/173.cpp) |
 | 172 | [Factorial Trailing Zeroes]                                  | [C](src/172.c)     |
 | 171 | [Excel Sheet Column Number]                                  | [C](src/171.c)     |
 | 170 | [Two Sum III - Data structure design] ☢                      |                    |

src/173.cpp

@@ -0,0 +1,71 @@
+#include <iostream>
+#include <stack>
+
+using namespace std;
+
+struct TreeNode {
+    int val;
+    TreeNode *left;
+    TreeNode *right;
+    TreeNode(int x) : val(x), left(NULL), right(NULL) {}
+};
+
+class BSTIterator {
+    stack<TreeNode *> path;
+public:
+    BSTIterator(TreeNode *root) {
+        while (root) {
+            path.push(root);
+            root = root->left;
+        }
+    }
+
+    /** @return whether we have a next smallest number */
+    bool hasNext() {
+        return !path.empty();
+    }
+
+    /** @return the next smallest number */
+    int next() {
+        int ans = -1;
+        if (hasNext()) {
+            TreeNode *p = path.top();
+            path.pop();
+            ans = p->val;
+            p = p->right;
+            while (p) {
+                path.push(p);
+                p = p->left;
+            }
+        }
+        return ans;
+    }
+};
+
+int main() {
+    /*
+     *        5
+     *      /   \
+     *     2     7
+     *    / \   / \
+     *   1   4  6  8
+     *      /
+     *     3
+     */
+
+    TreeNode *root = new TreeNode(5);
+    root->left = new TreeNode(2);
+    root->left->left = new TreeNode(1);
+    root->left->right = new TreeNode(4);
+    root->left->right->left = new TreeNode(3);
+
+    root->right = new TreeNode(7);
+    root->right->left = new TreeNode(6);
+    root->right->right = new TreeNode(8);
+
+    BSTIterator i = BSTIterator(root);
+    while (i.hasNext()) cout << i.next();
+    cout << endl;
+
+    return 0;
+}