LeetCode[232] 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).

push()就全都push到stack1中，执行pop()时，若stack2非空，直接执行stack2.pop()，若stack2为空，将stack1中元素全部压入stack2，再执行stack2.pop()。执行peek()时类似，就相当于top()

class Queue {public:// Push element x to the back of queue.void push(int x) {stack1.push(x);}// Removes the element from in front of queue.void pop(void) {if (stack2.empty()){while (!stack1.empty()){stack2.push(stack1.top());stack1.pop();}}if (!stack2.empty())stack2.pop();}// Get the front element.int peek(void) {if (stack2.empty()){while (!stack1.empty()){stack2.push(stack1.top());stack1.pop();}}if (!stack2.empty())return stack2.top();elsereturn 0;}// Return whether the queue is empty.bool empty(void) {return stack1.empty() && stack2.empty();}private:stack<int> stack1;stack<int> stack2;};