ImplementStackUsingQueues.cpp

// Source : https://leetcode.com/problems/implement-stack-using-queues/
// Author : Hao Chen
// Date : 2015-06-13

/********************************************************************************** 
 * 
 * Implement the following operations of a stack using queues.
 * 
 * push(x) -- Push element x onto stack.
 * 
 * pop() -- Removes the element on top of the stack.
 * 
 * top() -- Get the top element.
 * 
 * empty() -- Return whether the stack is empty.
 * 
 * Notes:
 * 
 * You must use only standard operations of a queue -- which means only push to back, 
 * peek/pop from front, size, and is empty operations are valid.
 *
 * Depending on your language, queue may not be supported natively. You may simulate 
 * a queue by using a list or deque (double-ended queue), as long as you use only 
 * standard operations of a queue.
 *
 * You may assume that all operations are valid (for example, no pop or top operations 
 * will be called on an empty stack).
 * 
 * Update (2015-06-11):
 * The class name of the Java function had been updated to MyStack instead of Stack.
 * 
 * Credits:Special thanks to @jianchao.li.fighter for adding this problem and all test cases.
 * 
 **********************************************************************************/


class Stack {
 public:
 // Push element x onto stack.
 void push(int x) {
 nums.push(x);
 }
 // Removes the element on top of the stack.
 void pop() {
 if (!empty()) {
 int len = nums.size()-1;
 //Don't treat the "-->" is a new operator. ;-)
 while ( len-->0) {
 nums.push(nums.front());
 nums.pop();
 }
 nums.pop();
 }
 }
 // Get the top element.
 int top() {
 return nums.back();
 }
 // Return whether the stack is empty.
 bool empty() {
 return nums.empty();
 }

 private:
 queue<int> nums;
};