leetcode解题: Implement Stack using Queues (225)

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Implement the following operations of a stack using queues.

push(x) – Push element x onto stack.
pop() – Removes the element on top of the stack.
top() – Get the top element.
empty() – Return whether the stack is empty.
Notes:
You must use only standard operations of a queue – which means only push to back, peek/pop from front, size, and is empty operations are valid.
Depending on your language, queue may not be supported natively. You may simulate a queue by using a list or deque (double-ended queue), as long as you use only standard operations of a queue.
You may assume that all operations are valid (for example, no pop or top operations will be called on an empty stack).

解法1：

基本思路是当执行top或者是pop的时候，我们需要取出最后一个元素。那么前面的元素放哪里？ 可以直接push回当前的queue。 这样，当除最后一个元素被重新push后，第一个就是我们要求的元素。
要注意的是，top是不改变数据的，所以执行完top之前，要把第一个元素push回queue
C++

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class Stack {
public:
    // Push element x onto stack.
    void push(int x) {
        q.push(x);
    }
    // Removes the element on top of the stack.
    void pop() {
        int s = q.size();
        for (int i = 0; i < s - 1; ++i) {
            q.push(q.front());
            q.pop();
        }
        q.pop();
    }
    // Get the top element.
    int top() {
        int s = q.size();
        for (int i = 0; i < s - 1; ++i) {
            q.push(q.front());
            q.pop();
        }
        int res = q.front();
        q.push(res);
        q.pop();
        return res;
    }
    // Return whether the stack is empty.
    bool empty() {
        return q.empty();
    }
private:
    queue<int> q;
};

Java

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