40 lines
1.5 KiB
Markdown
40 lines
1.5 KiB
Markdown
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Implement a last-in-first-out (LIFO) stack using only two queues. The implemented stack should support all the functions of a normal stack (`push`, `top`, `pop`, and `empty`).
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Implement the `MyStack` class:
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* `void push(int x)` Pushes element x to the top of the stack.
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* `int pop()` Removes the element on the top of the stack and returns it.
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* `int top()` Returns the element on the top of the stack.
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* `boolean empty()` Returns `true` if the stack is empty, `false` otherwise.
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**Notes:**
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* You must use **only** standard operations of a queue, which means that only `push to back`, `peek/pop from front`, `size` and `is empty` operations are valid.
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* 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.
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**Example 1:**
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Input
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["MyStack", "push", "push", "top", "pop", "empty"]
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[[], [1], [2], [], [], []]
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Output
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[null, null, null, 2, 2, false]
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Explanation
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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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**Constraints:**
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* `1 <= x <= 9`
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* At most `100` calls will be made to `push`, `pop`, `top`, and `empty`.
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* All the calls to `pop` and `top` are valid.
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**Follow-up:** Can you implement the stack using only one queue?
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https://leetcode.com/problems/implement-stack-using-queues/
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