栈的实现

• 225. Implement Stack using Queues
  • 思路
  • C++解法
  • Java解法

225. Implement Stack using Queues

Implement a last-in-first-out (LIFO) stack using only two queues. The implemented stack should support all the functions of a normal stack (push, top, pop, and empty).

Implement the MyStack class:

• void push(int x) Pushes element x to the top of the stack.
• int pop() Removes the element on the top of the stack and returns it.
• int top() Returns the element on the top of the stack.
• boolean empty() Returns true if the stack is empty, false otherwise.

Notes:

• You must use only standard operations of a queue, which means that only push to back, peek/pop from front, size and is empty operations are valid.
• Depending on your language, the queue may not be supported natively. You may simulate a queue using a list or deque (double-ended queue) as long as you use only a queue's standard operations.

Example 1:

Input

["MyStack", "push", "push", "top", "pop", "empty"]
[[], [1], [2], [], [], []]

Output

[null, null, null, 2, 2, false]

Explanation

MyStack myStack = new MyStack();
myStack.push(1);
myStack.push(2);
myStack.top(); // return 2
myStack.pop(); // return 2
myStack.empty(); // return False

Constraints:

• 1 <= x <= 9
• At most 100 calls will be made to push, pop, top, and empty.
• All the calls to pop and top are valid.

Follow-up: Can you implement the stack using only one queue?

思路

使用一个队列或两个队列都可以，使用一个队列时，弹出操作需要先弹出size-1个元素再加入，再弹出所要弹出的元素。

C++解法

class MyStack {
public:
    queue<int> q;
    MyStack() {
        
    }
    
    void push(int x) {
        q.push(x);
    }
    
    int pop() {
        int size = q.size();
        size--;
        while(size--){
            q.push(q.front());
            q.pop();
        }
        int result = q.front();
        q.pop();
        return result;
    }
    
    int top() {
        return q.back();
    }
    
    bool empty() {
        return q.size() == 0;
    }
};

/**
 * 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();
 */

Java解法