Leetcode 232, queue with stack

232、 Using stack to realize queue

1） Title Description

Please use only two stacks to implement the FIFO queue . The queue should support all operations supported by the general queue （push、pop、peek、empty）：

Realization MyQueue class ：

• void push(int x) Put the element x Push to the end of the queue
• int pop() Remove from the beginning of the queue and return the element
• int peek() Returns the element at the beginning of the queue
• boolean empty() If the queue is empty , return true ; otherwise , return false

explain ：

• you Can only Use standard stack operations —— It's just push to top, peek/pop from top, size, and is empty Operation is legal .
• The language you use may not support stacks . You can use list perhaps deque（ deque ） To simulate a stack , As long as it's a standard stack operation .

Example 1：

 Input ：
["MyQueue", "push", "push", "peek", "pop", "empty"]
[[], [1], [2], [], [], []]
 Output ：
[null, null, null, 1, 1, false]

 explain ：
MyQueue myQueue = new MyQueue();
myQueue.push(1); // queue is: [1]
myQueue.push(2); // queue is: [1, 2] (leftmost is front of the queue)
myQueue.peek(); // return 1
myQueue.pop(); // return 1, queue is [2]
myQueue.empty(); // return false

Tips ：

• 1 <= x <= 9
• Call at most 100 Time push、pop、peek and empty
• Suppose all operations are valid （ for example , An empty queue will not call pop perhaps peek operation ）

2） analysis

Because the stack is first in and last out , The queue is first in, first out , Two stacks can be used to exchange the order .

3）C++ Code

class MyQueue {
    
public:
    stack<int> s1;
    stack<int> s2;
    MyQueue() {
    
        
    }
    
    void push(int x) {
    
        s1.push(x);
    }
    
    int pop() {
    
        if(s2.empty()){
    
            while(!s1.empty()){
    
                s2.push(s1.top());
                s1.pop();
            }
        }
        int res=s2.top();
        s2.pop();
        return res;
    }
    
    int peek() {
    
        if(s2.empty()){
    
            while(!s1.empty()){
    
                s2.push(s1.top());
                s1.pop();
            }
        }
        int res=s2.top();
        return res;
    }
    
    bool empty() {
    
        return s1.empty()&&s2.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(); */