LeetCode 225. Implement Stack using Queues–用隊列實現棧–C++解法
LeetCode題解專欄:LeetCode題解
我做的所有的LeetCode的題目都放在這個專欄裏,大部分題目Java和Python的解法都有。
題目地址:Implement Stack using Queues - LeetCode
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, 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).
這道題目是經典的用隊列實現棧,雖然挺蠢的。
使用隊列,入隊或出隊,至少有一個的時間複雜度要爲O(n)。
使用鏈表的話可以做到O(1)。
C++解法如下:
class MyStack {
public:
queue<int> que;
/** Initialize your data structure here. */
MyStack() {
}
/** Push element x onto stack. */
void push(int x) {
que.push(x);
for (int i = 0; i < que.size() - 1; i++) {
que.push(que.front());
que.pop();
}
}
/** Removes the element on top of the stack and returns that element. */
int pop() {
int front = top();
que.pop();
return front;
}
/** Get the top element. */
int top() {
return que.front();
}
/** Returns whether the stack is empty. */
bool empty() {
return que.empty();
}
};
/**
* 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();
*/