114 lines
2.7 KiB
Markdown
114 lines
2.7 KiB
Markdown
# Leetcode Implement-Queue-Using-Stacks
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#### 2022-07-03 09:52
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> ##### Data structures:
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> #DS #stack #queue
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> ##### Difficulty:
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> #coding_problem #difficulty-easy
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> ##### Additional tags:
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> #leetcode #CS_list_need_understanding
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> ##### Revisions:
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> N/A
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##### Related topics:
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```expander
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tag:#stack tag:#queue
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```
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##### Links:
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- [Link to problem](https://leetcode.com/problems/implement-queue-using-stacks/submissions/)
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___
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### Problem
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Implement a first in first out (FIFO) queue using only two stacks. The implemented queue should support all the functions of a normal queue (`push`, `peek`, `pop`, and `empty`).
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Implement the `MyQueue` class:
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- `void push(int x)` Pushes element x to the back of the queue.
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- `int pop()` Removes the element from the front of the queue and returns it.
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- `int peek()` Returns the element at the front of the queue.
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- `boolean empty()` Returns `true` if the queue is empty, `false` otherwise.
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**Notes:**
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- 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.
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- Depending on your language, the stack may not be supported natively. You may simulate a stack using a list or deque (double-ended queue) as long as you use only a stack's standard operations.
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#### Examples
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**Input**
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```
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["MyQueue", "push", "push", "peek", "pop", "empty"]
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[[], [1], [2], [], [], []]
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```
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**Output**
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[null, null, null, 1, 1, false]
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**Explanation**
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MyQueue myQueue = new MyQueue();
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myQueue.push(1); // queue is: [1]
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myQueue.push(2); // queue is: [1, 2] (leftmost is front of the queue)
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myQueue.peek(); // return 1
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myQueue.pop(); // return 1, queue is [2]
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myQueue.empty(); // return false
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#### Constraints
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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`, `peek`, and `empty`.
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- All the calls to `pop` and `peek` are valid.
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### Thoughts
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> [!summary]
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> Use two stacks to implement a queue.
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One stack reverse the FIFO order, but two stack can reverse back.
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In real life, this can improve speed as you only lock one stack because of thread safety.
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### Solution
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```cpp
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class MyQueue {
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stack<int> input;
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stack<int> out;
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public:
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void push(int x) { input.push(x); }
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int pop() {
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move();
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int top = out.top();
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out.pop();
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return top;
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}
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int peek() {
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move();
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return out.top();
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}
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void move() {
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if (out.empty()) {
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while (!input.empty()) {
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out.push(input.top());
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input.pop();
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}
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}
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}
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bool empty() { return input.empty() && out.empty(); }
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};
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/**
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* Your MyQueue object will be instantiated and called as such:
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* MyQueue* obj = new MyQueue();
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* obj->push(x);
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* int param_2 = obj->pop();
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* int param_3 = obj->peek();
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* bool param_4 = obj->empty();
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*/
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``` |