232

232. Implement Queue using Stacks (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).

Related Topics:
Stack, Design

Similar Questions:

• Implement Stack using Queues (Easy)

Solution 1.

// OJ: https://leetcode.com/problems/implement-queue-using-stacks
// Author: github.com/lzl124631x
// Time: O(1) amortized.
// Space: O(1)
class MyQueue {
    stack<int> in, out;
public:
    MyQueue() {}
    
    void push(int x) {
        in.push(x);
    }
    
    int pop() {
        int val = peek();
        out.pop();
        return val;
    }
    
    int peek() {
        if (out.empty()) {
            while (in.size()) {
                out.push(in.top());
                in.pop();
            }
        }
        return out.top();
    }
    
    bool empty() {
        return in.empty() && out.empty();
    }
};