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 emptyoperations 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).        

用栈来实现队列的功能

C++（3ms）：  两个栈

class MyQueue {
public:
    stack<int> st1 ;
    stack<int> st2 ;
    /** Initialize your data structure here. */
    MyQueue() {
        
    }
    
    /** Push element x to the back of queue. */
    void push(int x) {
        st1.push(x) ;
    }
    
    /** Removes the element from in front of queue and returns that element. */
    int pop() {
        move();
        int head = st2.top() ;
        st2.pop() ;
        return head ;
    }
    
    /** Get the front element. */
    int peek() {
        move();
        return st2.top() ;
    }
    
    /** Returns whether the queue is empty. */
    bool empty() {
        return st1.empty() && st2.empty() ;
    }
    
    void move(){
        if (st2.size() <= 0){
            while(st1.size() > 0){
                int t = st1.top() ;
                st1.pop() ;
                st2.push(t) ;
            }
        }
    }
};

/**
 * 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();
 */

C++(0ms):   一个栈

class MyQueue {
public:
    stack<int> st; 
    /** Initialize your data structure here. */
    MyQueue() {
        
    }
    
    /** Push element x to the back of queue. */
    void push(int x) {
        pushHelper(x) ;
    }
    
    void pushHelper(int x){
        if (st.size() == 0){
            st.push(x) ;
            return ;
        }
        int t = st.top() ;
        st.pop() ;
        pushHelper(x) ;
        st.push(t) ;
        return ;
    }
    
    /** Removes the element from in front of queue and returns that element. */
    int pop() {
        int t = st.top() ;
        st.pop() ;
        return t ;
    }
    
    /** Get the front element. */
    int peek() {
        return st.top() ;
    }
    
    /** Returns whether the queue is empty. */
    bool empty() {
        return st.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();
 */