implement-queue-using-stacks(AC).cpp

// 1AC, all O(1)
#include <stack>
using namespace std;

class Queue {
public:
    // Push element x to the back of queue.
    void push(int x) {
        s1.push(x);
    }

    // Removes the element from in front of queue.
    void pop() {
        if (s2.empty()) {
            while (!s1.empty()) {
                s2.push(s1.top());
                s1.pop();
            }
        }
        int val = s2.top();
        s2.pop();
    }

    // Get the front element.
    int peek() {
        if (s2.empty()) {
            while (!s1.empty()) {
                s2.push(s1.top());
                s1.pop();
            }
        }
        return s2.top();
    }

    // Return whether the queue is empty.
    bool empty() {
        return s1.empty() && s2.empty();
    }
protected:
    stack<int> s1, s2;
};