232. Implement Queue using Stacks.ipynb

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    "Implement the following operations of a queue using stacks.\n",
    "\n",
    "push(x) -- Push element x to the back of queue.\n",
    "pop() -- Removes the element from in front of queue.\n",
    "peek() -- Get the front element.\n",
    "empty() -- Return whether the queue is empty.\n",
    "Notes:\n",
    "You must use only standard operations of a stack -- which means only push to top, peek/pop from top, size, and is empty operations are valid.\n",
    "Depending on your language, stack may not be supported natively. You may simulate a stack by using a list or deque (double-ended queue), as long as you use only standard operations of a stack.\n",
    "You may assume that all operations are valid (for example, no pop or peek operations will be called on an empty queue)."
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    "class MyQueue(object):\n",
    "\n",
    "    def __init__(self):\n",
    "        \"\"\"\n",
    "        Initialize your data structure here.\n",
    "        \"\"\"\n",
    "        self.in_stack=[]\n",
    "        self.out_stack=[]\n",
    "\n",
    "    def push(self, x):\n",
    "        \"\"\"\n",
    "        Push element x to the back of queue.\n",
    "        :type x: int\n",
    "        :rtype: void\n",
    "        \"\"\"\n",
    "        self.in_stack.append(x)\n",
    "\n",
    "    def pop(self):\n",
    "        \"\"\"\n",
    "        Removes the element from in front of queue and returns that element.\n",
    "        :rtype: int\n",
    "        \"\"\"\n",
    "        if self.empty():\n",
    "            return None\n",
    "        if self.out_stack:\n",
    "            return self.out_stack.pop()\n",
    "        else:\n",
    "            while(self.in_stack):\n",
    "                self.out_stack.append(self.in_stack.pop())\n",
    "            return self.out_stack.pop()\n",
    "        \n",
    "\n",
    "    def peek(self):\n",
    "        \"\"\"\n",
    "        Get the front element.\n",
    "        :rtype: int\n",
    "        \"\"\"\n",
    "        if self.empty():\n",
    "            return None\n",
    "        if self.out_stack:\n",
    "            return self.out_stack[-1]\n",
    "        else:\n",
    "            while(self.in_stack):\n",
    "                self.out_stack.append(self.in_stack.pop())\n",
    "                \n",
    "            return self.out_stack[-1]\n",
    "        \n",
    "\n",
    "    def empty(self):\n",
    "        \"\"\"\n",
    "        Returns whether the queue is empty.\n",
    "        :rtype: bool\n",
    "        \"\"\"\n",
    "        return not self.in_stack and not self.out_stack\n",
    "        \n",
    "\n",
    "\n",
    "# Your MyQueue object will be instantiated and called as such:\n",
    "# obj = MyQueue()\n",
    "# obj.push(x)\n",
    "# param_2 = obj.pop()\n",
    "# param_3 = obj.peek()\n",
    "# param_4 = obj.empty()"
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    "obj = MyQueue()"
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    "obj.push(2)"
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    "p2=obj.pop()"
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   "source": [
    "obj.peek()"
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