【LeetCode】225. Implement Stack using Queues

標籤：

Implement Stack using Queues

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.

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).

 

Update (2015-06-11):
The class name of the Java function had been updated to MyStack instead of Stack.

Credits:
Special thanks to @jianchao.li.fighter for adding this problem and all test cases.

 

兩個棧實現隊列：

一個棧存放當前所有元素，另一個棧為空白。

push: 元素進非空棧，並更新top值為當前進隊元素。

pop: 將非空棧的值，除了最後一個(待出隊)，其餘的裝入空棧。注意更新top值為最後第二個進隊元素。

top: 直接返回即時更新的top值

empty: 判斷兩個棧是否均為空白

class Stack {public:    queue<int> q1;    queue<int> q2;    int n;        // Push element x onto stack.    void push(int x) {        if(q2.empty())            q1.push(x);        else            q2.push(x);        n = x;    }    // Removes the element on top of the stack.    void pop() {        if(!q1.empty())        {            while(q1.size() > 1)            {                int front = q1.front();                if(q1.size() == 2)                    n = front;                q1.pop();                q2.push(front);            }            q1.pop();        }        else        {            while(q2.size() > 1)            {                int front = q2.front();                if(q2.size() == 2)                    n = front;                q2.pop();                q1.push(front);            }            q2.pop();        }    }    // Get the top element.    int top() {        return n;    }    // Return whether the stack is empty.    bool empty() {        return q1.empty() && q2.empty();    }};

【LeetCode】225. Implement Stack using Queues