225. Implement Stack using Queues

类似问题：232. Implement Queue using Stacks

问题：

设计数据结构，使用queue实现stack。

实现以下功能：

• 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.

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.

　　

解法：单队列->实现队列

参考labuladong解析

 

 

重点在：

查栈顶元素top 和 出栈pop

使用额外变量，记录top元素：top_ele

循环queue内所有元素，剩下最后一个即为要求栈顶元素top

同时更新 top_ele 为倒数第二个元素。

 

参考代码：

class MyStack {
public:
    /** Initialize your data structure here. */
    MyStack() {
        
    }
    
    /** Push element x onto stack. */
    void push(int x) {
        q.push(x);
        top_ele = x;
    }
    
    /** Removes the element on top of the stack and returns that element. */
    int pop() {
        int size = q.size();
        while(size>2) {
            q.push(q.front());
            q.pop();
            size--;
        }
        top_ele = q.front();
        q.pop();
        q.push(top_ele);
        int res = q.front();
        q.pop();
        return res;
    }
    
    /** Get the top element. */
    int top() {
        return top_ele;
    }
    
    /** Returns whether the stack is empty. */
    bool empty() {
        return q.empty();
    }
private:
    queue<int> q;
    int top_ele;
};

/**
 * 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();
 */