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49 lines
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49 lines
2.0 KiB
HTML
<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>
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<p>Implement the <code>MyStack</code> class:</p>
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<ul>
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<li><code>void push(int x)</code> Pushes element x to the top of the stack.</li>
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<li><code>int pop()</code> Removes the element on the top of the stack and returns it.</li>
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<li><code>int top()</code> Returns the element on the top of the stack.</li>
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<li><code>boolean empty()</code> Returns <code>true</code> if the stack is empty, <code>false</code> otherwise.</li>
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</ul>
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<p><b>Notes:</b></p>
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<ul>
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<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>
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<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's standard operations.</li>
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</ul>
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<p> </p>
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<p><strong>Example 1:</strong></p>
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<pre>
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<strong>Input</strong>
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["MyStack", "push", "push", "top", "pop", "empty"]
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[[], [1], [2], [], [], []]
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<strong>Output</strong>
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[null, null, null, 2, 2, false]
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<strong>Explanation</strong>
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MyStack myStack = new MyStack();
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myStack.push(1);
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myStack.push(2);
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myStack.top(); // return 2
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myStack.pop(); // return 2
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myStack.empty(); // return False
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</pre>
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<p> </p>
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<p><strong>Constraints:</strong></p>
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<ul>
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<li><code>1 <= x <= 9</code></li>
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<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>
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<li>All the calls to <code>pop</code> and <code>top</code> are valid.</li>
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</ul>
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<p> </p>
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<p><strong>Follow-up:</strong> Can you implement the stack using only one queue?</p>
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