LeetCode --- 232. Implement Queue using Stacks

232. Implement Queue using Stacks

Difficulty: Easy

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.

Example:

MyQueue queue = new MyQueue();

queue.push(1);
queue.push(2);  
queue.peek();  // returns 1
queue.pop();   // returns 1
queue.empty(); // returns false```

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).

思路

使用两个栈，s1,s2一个为输入，一个为输出。具体如下。

Solution

Language: C++

class MyQueue {
    stack s1, s2;
public: 
    /** Initialize your data structure here. */
    MyQueue() {
       
    }
    /** Push element x to the back of queue. */
    void push(int x) {
        s1.push(x);
    }
    
    /** Removes the element from in front of queue and returns that element. */
    int pop() {
        if (s2.empty()){
            while (!s1.empty()){
                s2.push(s1.top());
                s1.pop();
            }
        }
        int num = s2.top();
        s2.pop();
        return num;
    }
    
    /** Get the front element. */
    int peek() {
        if (s2.empty()){
            while (!s1.empty()){
                s2.push(s1.top());
                s1.pop();
            }
        }
        return s2.top();
    }
    
    /** Returns whether the queue is empty. */
    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();
 */