[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 onlypush 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 {private:        stack<int>stk1,stk2;public:    // Push element x to the back of queue.    /*stack is LIFO, but queue is FIFO, so always       keep element FIFO order in stack      so push() O(n), pop(),peek(),empty() O(1) operation    */    void push(int x) {        while(!stk1.empty()){            stk2.push(stk1.top());            stk1.pop();        }        stk1.push(x);        while(!stk2.empty()){            stk1.push(stk2.top());            stk2.pop();        }    }    // Removes the element from in front of queue.    void pop(void) {        if(!stk1.empty())stk1.pop();    }    // Get the front element.    int peek(void) {        return stk1.top();    }    // Return whether the queue is empty.    bool empty(void) {        return stk1.empty();    }};