LeetCode|Implement Queue using Stacks

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 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).

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(void) {        if(!s2.empty()){            s2.pop();        } else{            while(!s1.empty()){                s2.push(s1.top());                s1.pop();            } s2.pop();        }    }    // Get the front element.    int peek(void) {        if(!s2.empty()){            return s2.top();        } else{            while(!s1.empty()){                s2.push(s1.top());                s1.pop();            } return s2.top();        }    }    // Return whether the queue is empty.    bool empty(void) {        return s1.empty() && s2.empty();    }    stack<int> s1, s2;};