Splice ./universe directory into ./

Manually merging:
- README.md: I added the description from universe/README.md into the heading of
  dotfiles/README.md.
- .envrc: dotfiles/.envrc was a superset of universe/.envrc
- .gitignore: Adding some of the ignored patterns from universe/.gitignore to
  dotfiles/.gitignore

Everything else here should be a simple rename.

1 files changed, 107 insertions, 0 deletions

diff --git a/data_structures_and_algorithms/largest-stack.py b/data_structures_and_algorithms/largest-stack.py
new file mode 100644
index 000000000000..aab9671eb6d3
--- /dev/null
+++ b/data_structures_and_algorithms/largest-stack.py
@@ -0,0 +1,107 @@
+import unittest
+
+
+class Stack(object):
+    def __init__(self):
+        """Initialize an empty stack"""
+        self.items = []
+
+    def push(self, item):
+        """Push a new item onto the stack"""
+        self.items.append(item)
+
+    def pop(self):
+        """Remove and return the last item"""
+        # If the stack is empty, return None
+        # (it would also be reasonable to throw an exception)
+        if not self.items:
+            return None
+
+        return self.items.pop()
+
+    def peek(self):
+        """Return the last item without removing it"""
+        if not self.items:
+            return None
+        return self.items[-1]
+
+
+class MaxStack(object):
+    # Implement the push, pop, and get_max methods
+    def __init__(self):
+        self.m = Stack()
+        self.stack = Stack()
+
+    def push(self, item):
+        if self.m.peek() is None:
+            self.m.push(item)
+        elif item >= self.m.peek():
+            self.m.push(item)
+        self.stack.push(item)
+
+    def pop(self):
+        x = self.stack.pop()
+        if x == self.m.peek():
+            self.m.pop()
+        return x
+
+    def get_max(self):
+        return self.m.peek()
+
+
+################################################################################
+# Tests
+################################################################################
+class Test(unittest.TestCase):
+    def test_stack_usage(self):
+        max_stack = MaxStack()
+
+        max_stack.push(5)
+
+        actual = max_stack.get_max()
+        expected = 5
+        self.assertEqual(actual, expected)
+
+        max_stack.push(4)
+        max_stack.push(7)
+        max_stack.push(7)
+        max_stack.push(8)
+
+        actual = max_stack.get_max()
+        expected = 8
+        self.assertEqual(actual, expected)
+
+        actual = max_stack.pop()
+        expected = 8
+        self.assertEqual(actual, expected)
+
+        actual = max_stack.get_max()
+        expected = 7
+        self.assertEqual(actual, expected)
+
+        actual = max_stack.pop()
+        expected = 7
+        self.assertEqual(actual, expected)
+
+        actual = max_stack.get_max()
+        expected = 7
+        self.assertEqual(actual, expected)
+
+        actual = max_stack.pop()
+        expected = 7
+        self.assertEqual(actual, expected)
+
+        actual = max_stack.get_max()
+        expected = 5
+        self.assertEqual(actual, expected)
+
+        actual = max_stack.pop()
+        expected = 4
+        self.assertEqual(actual, expected)
+
+        actual = max_stack.get_max()
+        expected = 5
+        self.assertEqual(actual, expected)
+
+
+unittest.main(verbosity=2)