Leetcode 232. Implement Queue using Stacks

Implement the following operations of a queue using stacks.

• push(x) -- Push element x to the back of queue.
• pop() -- Removes the element from in front of queue.
• peek() -- Get the front element.
• empty() -- Return whether the queue is empty.

Notes:

• 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.
• Depending on your language, stack may not be supported natively. You may simulate a stack by using a list or deque (double-ended queue), as long as you use only standard operations of a stack.
• You may assume that all operations are valid (for example, no pop or peek operations will be called on an empty queue).

用棧實現隊列。

可以用兩個棧實現，也可以像我這樣顯式地用一個棧實現，其實本質還是兩個棧。

關鍵在於push操作如何讓新的元素放入棧底，顯然可以用遞歸的方式。

class MyQueue {
public:
    stack<int> s;
    /** Initialize your data structure here. */
    MyQueue() {
        
    }
    
    /** Push element x to the back of queue. */
    void push(int x) {
        if(s.empty()) 
        {
            s.push(x);
            return ;
        }
        int tmp = pop();
        push(x);
        s.push(tmp);
    }
    
    /** Removes the element from in front of queue and returns that element. */
    int pop() {
        int res = s.top();
        s.pop();
        return res;
    }
    
    /** Get the front element. */
    int peek() {
        return s.top();
    }
    
    /** Returns whether the queue is empty. */
    bool empty() {
        return s.empty();
    }
};

/**
 * Your MyQueue object will be instantiated and called as such:
 * MyQueue obj = new MyQueue();
 * obj.push(x);
 * int param_2 = obj.pop();
 * int param_3 = obj.peek();
 * bool param_4 = obj.empty();
 */