feat(leetcode): add No.225

Author: mickey0524

225.Implement-Stack-using-Queues.py

@@ -0,0 +1,69 @@
+# https://leetcode.com/problems/implement-stack-using-queues/
+#
+# algorithms
+# Medium (39.64%)
+# Total Accepted:    132,639
+# Total Submissions: 334,578
+# beats 94.52% of python submissions
+
+from collections import deque
+
+
+class MyStack(object):
+
+    def __init__(self):
+        """
+        Initialize your data structure here.
+        """
+        self.q_1 = deque()
+        self.q_2 = deque()
+        self.use_1 = True
+
+    def push(self, x):
+        """
+        Push element x onto stack.
+        :type x: int
+        :rtype: void
+        """
+        if self.use_1:
+            self.q_1.appendleft(x)
+        else:
+            self.q_2.appendleft(x)
+
+    def pop(self):
+        """
+        Removes the element on top of the stack and returns that element.
+        :rtype: int
+        """
+        return self._pop_or_top(True)
+
+    def top(self):
+        """
+        Get the top element.
+        :rtype: int
+        """
+        return self._pop_or_top(False)
+
+    def _pop_or_top(self, is_pop):
+        if self.use_1:
+            from_q, to_q = self.q_1, self.q_2
+        else:
+            from_q, to_q = self.q_2, self.q_1
+
+        while len(from_q) > 1:
+            to_q.appendleft(from_q.pop())
+
+        res = from_q.pop()
+        
+        if not is_pop:
+            to_q.appendleft(res)
+        self.use_1 = not self.use_1
+        
+        return res
+
+    def empty(self):
+        """
+        Returns whether the stack is empty.
+        :rtype: bool
+        """
+        return len(self.q_1) + len(self.q_2) == 0