implement-stack-using-queues

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Algorithms

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>).

Implement the <code>MyStack</code> class:

<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>

Notes:

<ul>
	<li>You must use only 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>

&nbsp;
Example 1:

<pre>
Input
[&quot;MyStack&quot;, &quot;push&quot;, &quot;push&quot;, &quot;top&quot;, &quot;pop&quot;, &quot;empty&quot;]
[[], [1], [2], [], [], []]
Output
[null, null, null, 2, 2, false]

Explanation
MyStack myStack = new MyStack();
myStack.push(1);
myStack.push(2);
myStack.top(); // return 2
myStack.pop(); // return 2
myStack.empty(); // return False
</pre>

&nbsp;
Constraints:

<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>

&nbsp;
Follow-up: Can you implement the stack using only one queue?

Implement Stack using Queues

Implement Queue using Stacks

implement-queue-using-stacks

用栈实现队列

Stack

Design

Queue

&#35831;&#20320;&#20165;&#20351;&#29992;&#20004;&#20010;&#38431;&#21015;&#23454;&#29616;&#19968;&#20010;&#21518;&#20837;&#20808;&#20986;&#65288;LIFO&#65289;&#30340;&#26632;&#65292;&#24182;&#25903;&#25345;&#26222;&#36890;&#26632;&#30340;&#20840;&#37096;&#22235;&#31181;&#25805;&#20316;&#65288;<code>push</code>&#12289;<code>top</code>&#12289;<code>pop</code> &#21644; <code>empty</code>&#65289;&#12290;

&#23454;&#29616; <code>MyStack</code> &#31867;&#65306;

<ul>
	<li><code>void push(int x)</code> &#23558;&#20803;&#32032; x &#21387;&#20837;&#26632;&#39030;&#12290;</li>
	<li><code>int pop()</code> &#31227;&#38500;&#24182;&#36820;&#22238;&#26632;&#39030;&#20803;&#32032;&#12290;</li>
	<li><code>int top()</code> &#36820;&#22238;&#26632;&#39030;&#20803;&#32032;&#12290;</li>
	<li><code>boolean empty()</code> &#22914;&#26524;&#26632;&#26159;&#31354;&#30340;&#65292;&#36820;&#22238; <code>true</code> &#65307;&#21542;&#21017;&#65292;&#36820;&#22238; <code>false</code> &#12290;</li>
</ul>

&nbsp;

&#27880;&#24847;&#65306;

<ul>
	<li>&#20320;&#21482;&#33021;&#20351;&#29992;&#38431;&#21015;&#30340;&#26631;&#20934;&#25805;&#20316; &mdash;&mdash; &#20063;&#23601;&#26159;&nbsp;<code>push to back</code>&#12289;<code>peek/pop from front</code>&#12289;<code>size</code> &#21644;&nbsp;<code>is empty</code>&nbsp;&#36825;&#20123;&#25805;&#20316;&#12290;</li>
	<li>&#20320;&#25152;&#20351;&#29992;&#30340;&#35821;&#35328;&#20063;&#35768;&#19981;&#25903;&#25345;&#38431;&#21015;&#12290;&nbsp;&#20320;&#21487;&#20197;&#20351;&#29992; list &#65288;&#21015;&#34920;&#65289;&#25110;&#32773; deque&#65288;&#21452;&#31471;&#38431;&#21015;&#65289;&#26469;&#27169;&#25311;&#19968;&#20010;&#38431;&#21015;&nbsp;, &#21482;&#35201;&#26159;&#26631;&#20934;&#30340;&#38431;&#21015;&#25805;&#20316;&#21363;&#21487;&#12290;</li>
</ul>

&nbsp;

&#31034;&#20363;&#65306;

<pre>
&#36755;&#20837;&#65306;
["MyStack", "push", "push", "top", "pop", "empty"]
[[], [1], [2], [], [], []]
&#36755;&#20986;&#65306;
[null, null, null, 2, 2, false]

&#35299;&#37322;&#65306;
MyStack myStack = new MyStack();
myStack.push(1);
myStack.push(2);
myStack.top(); // &#36820;&#22238; 2
myStack.pop(); // &#36820;&#22238; 2
myStack.empty(); // &#36820;&#22238; False
</pre>

&nbsp;

&#25552;&#31034;&#65306;

<ul>
	<li><code>1 &lt;= x &lt;= 9</code></li>
	<li>&#26368;&#22810;&#35843;&#29992;<code>100</code> &#27425; <code>push</code>&#12289;<code>pop</code>&#12289;<code>top</code> &#21644; <code>empty</code></li>
	<li>&#27599;&#27425;&#35843;&#29992; <code>pop</code> &#21644; <code>top</code> &#37117;&#20445;&#35777;&#26632;&#19981;&#20026;&#31354;</li>
</ul>

&nbsp;

&#36827;&#38454;&#65306;&#20320;&#33021;&#21542;&#20165;&#29992;&#19968;&#20010;&#38431;&#21015;&#26469;&#23454;&#29616;&#26632;&#12290;

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