From 273d0efcb4aa117b1e334f4c4b010cbbd854b7dd Mon Sep 17 00:00:00 2001
From: Donne Martin <donne.martin@gmail.com>
Date: Sat, 23 May 2015 17:01:45 -0400
Subject: [PATCH] Added stack method is_empty().

---
 stacks-queues/stack.ipynb | 26 ++++++++++++++++++++++----
 stacks-queues/stack.py    |  5 ++++-
 2 files changed, 26 insertions(+), 5 deletions(-)

diff --git a/stacks-queues/stack.ipynb b/stacks-queues/stack.ipynb
index 3aa5a35..f9d83ee 100644
--- a/stacks-queues/stack.ipynb
+++ b/stacks-queues/stack.ipynb
@@ -4,7 +4,7 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "## Problem: Implement a stack with push, pop, and peek methods using a linked list.\n",
+    "## Problem: Implement a stack with push, pop, peek, and is_empty methods using a linked list.\n",
     "\n",
     "* [Clarifying Questions](#Clarifying-Questions)\n",
     "* [Test Cases](#Test-Cases)\n",
@@ -42,7 +42,12 @@
     "### Peek\n",
     "\n",
     "* Peek on empty stack\n",
-    "* Peek on one or more element stack"
+    "* Peek on one or more element stack\n",
+    "\n",
+    "### Is Empty\n",
+    "\n",
+    "* Is empty on empty stack\n",
+    "* Is empty on one or more element stack"
    ]
   },
   {
@@ -80,6 +85,14 @@
     "\n",
     "Complexity:\n",
     "* Time: O(1)\n",
+    "* Space: O(1)\n",
+    "\n",
+    "### Is Empty\n",
+    "* If peek has a value, return False\n",
+    "* Else return True\n",
+    "\n",
+    "Complexity:\n",
+    "* Time: O(1)\n",
     "* Space: O(1)"
    ]
   },
@@ -124,7 +137,10 @@
     "    def peek(self):\n",
     "        if self.top is not None:\n",
     "            return self.top.data\n",
-    "        return None"
+    "        return None\n",
+    "\n",
+    "    def is_empty(self):\n",
+    "        return self.peek() is None"
    ]
   },
   {
@@ -164,8 +180,10 @@
     "print(stack.peek())\n",
     "print(stack.pop())\n",
     "print(stack.peek())\n",
+    "print(stack.is_empty())\n",
     "print(stack.pop())\n",
-    "print(stack.peek())"
+    "print(stack.peek())\n",
+    "print(stack.is_empty())"
    ]
   },
   {
diff --git a/stacks-queues/stack.py b/stacks-queues/stack.py
index e8780be..e1a9ddb 100644
--- a/stacks-queues/stack.py
+++ b/stacks-queues/stack.py
@@ -23,4 +23,7 @@ class Stack(object):
     def peek(self):
         if self.top is not None:
             return self.top.data
-        return None
\ No newline at end of file
+        return None
+
+    def is_empty(self):
+        return self.peek() is None
\ No newline at end of file