Create implement-stack-using-queues.py

Author: kamyu104

Python/implement-stack-using-queues.py

@@ -0,0 +1,64 @@
+# Time: push: O(n), pop: O(1), top: O(1)
+# Space: O(1)
+#
+# Implement the following operations of a stack using queues.
+#
+# push(x) -- Push element x onto stack.
+# pop() -- Removes the element on top of the stack.
+# top() -- Get the top element.
+# empty() -- Return whether the stack is empty.
+# Notes:
+# You must use only standard operations of a queue -- which
+# means only push to back, peek/pop from front, size, and is
+# empty operations are valid.
+# Depending on your language, queue may not be supported natively.
+# You may simulate a queue by using a list or deque (double-ended
+# queue), as long as you use only standard operations of a queue.
+# You may assume that all operations are valid (for example, no pop
+# or top operations will be called on an empty stack).
+#
+
+class Queue:
+    def __init__(self):
+        self.data = collections.deque()
+        
+    def push(self, x):
+        self.data.append(x)
+    
+    def peek(self):
+        return self.data[0]
+    
+    def pop(self):
+        return self.data.popleft()
+    
+    def size(self):
+        return len(self.data)
+    
+    def empty(self):
+        return len(self.data) == 0
+
+
+class Stack:
+    # initialize your data structure here.
+    def __init__(self):
+        self.q = Queue()
+
+    # @param x, an integer
+    # @return nothing
+    def push(self, x):
+        q = self.q
+        q.push(x)
+        for _ in xrange(q.size() - 1):
+            q.push(q.pop())
+
+    # @return nothing
+    def pop(self):
+        self.q.pop()
+
+    # @return an integer
+    def top(self):
+        return self.q.peek()
+
+    # @return an boolean
+    def empty(self):
+        return self.q.empty()