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.
Example:
MyStack stack = new MyStack(); stack.push(1); stack.push(2); stack.top(); // returns 2 stack.pop(); // returns 2 stack.empty(); // returns false
Notes:
- You must use only standard operations of a queue -- which means only
push to back
,peek/pop from front
,size
, andis 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).
A:
就是简单的,每次都pop()出来,再放到最后去了
class MyStack { public: /** Initialize your data structure here. */ MyStack() { } /** Push element x onto stack. */ void push(int x) { dq.push_back(x); } /** Removes the element on top of the stack and returns that element. */ int pop() { deque<int> dq2; while(dq.size()>1) { int v = dq.front(); dq.pop_front(); dq2.push_back(v); } int res = dq.front(); dq = dq2; return res; } /** Get the top element. */ int top() { deque<int> dq2; int res; while(dq.size()>0) { res = dq.front(); dq.pop_front(); dq2.push_back(res); } dq = dq2; return res; } /** Returns whether the stack is empty. */ bool empty() { return dq.empty(); } deque<int> dq; }; /** * Your MyStack object will be instantiated and called as such: * MyStack* obj = new MyStack(); * obj->push(x); * int param_2 = obj->pop(); * int param_3 = obj->top(); * bool param_4 = obj->empty(); */
Mistakes:
1: 第一遍的时候,理解成是用2个Stack来表示Queue了 太TMD搞笑(Diu Ren)了
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