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算法题（国外版）/implement-stack-using-queues.html

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+<p>Implement a last-in-first-out (LIFO) stack using only two queues. The implemented stack should support all the functions of a normal stack (<code>push</code>, <code>top</code>, <code>pop</code>, and <code>empty</code>).</p>
+
+<p>Implement the <code>MyStack</code> class:</p>
+
+<ul>
+	<li><code>void push(int x)</code> Pushes element x to the top of the stack.</li>
+	<li><code>int pop()</code> Removes the element on the top of the stack and returns it.</li>
+	<li><code>int top()</code> Returns the element on the top of the stack.</li>
+	<li><code>boolean empty()</code> Returns <code>true</code> if the stack is empty, <code>false</code> otherwise.</li>
+</ul>
+
+<p><b>Notes:</b></p>
+
+<ul>
+	<li>You must use <strong>only</strong> standard operations of a queue, which means that only <code>push to back</code>, <code>peek/pop from front</code>, <code>size</code> and <code>is empty</code> operations are valid.</li>
+	<li>Depending on your language, the queue may not be supported natively. You may simulate a queue using a list or deque (double-ended queue) as long as you use only a queue&#39;s standard operations.</li>
+</ul>
+
+<p>&nbsp;</p>
+<p><strong>Example 1:</strong></p>
+
+<pre>
+<strong>Input</strong>
+[&quot;MyStack&quot;, &quot;push&quot;, &quot;push&quot;, &quot;top&quot;, &quot;pop&quot;, &quot;empty&quot;]
+[[], [1], [2], [], [], []]
+<strong>Output</strong>
+[null, null, null, 2, 2, false]
+
+<strong>Explanation</strong>
+MyStack myStack = new MyStack();
+myStack.push(1);
+myStack.push(2);
+myStack.top(); // return 2
+myStack.pop(); // return 2
+myStack.empty(); // return False
+</pre>
+
+<p>&nbsp;</p>
+<p><strong>Constraints:</strong></p>
+
+<ul>
+	<li><code>1 &lt;= x &lt;= 9</code></li>
+	<li>At most <code>100</code> calls will be made to <code>push</code>, <code>pop</code>, <code>top</code>, and <code>empty</code>.</li>
+	<li>All the calls to <code>pop</code> and <code>top</code> are valid.</li>
+</ul>
+
+<p>&nbsp;</p>
+<p><strong>Follow-up:</strong> Can you implement the stack using only one queue?</p>