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.
- You must use only standard operations of a stack -- which means only
push to top
,peek/pop from top
,size
, andis 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).
假设两者均为空,说明队列为空。
class Queue { public: // Push element x to the back of queue. void push(int x) { push_stack.push(x); } // Removes the element from in front of queue. void pop(void) { if (pop_stack.empty()) { while (!push_stack.empty()) { pop_stack.push(push_stack.top()); push_stack.pop(); } if (!pop_stack.empty()) pop_stack.pop(); } else { pop_stack.pop(); } } // Get the front element. int peek(void) { if (pop_stack.empty()) { while (!push_stack.empty()) { pop_stack.push(push_stack.top()); push_stack.pop(); } if (!pop_stack.empty()) return pop_stack.top(); } else { return pop_stack.top(); } return 0; } // Return whether the queue is empty. bool empty(void) { if (pop_stack.empty() && push_stack.empty()) return true; else return false; } private: stack<int> pop_stack; stack<int> push_stack; };