[LeetCode] Implement Stack using Queues

标签：c++   leetcode   

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.

解题思路：

用两个队列来模拟栈。记录当前目标队列。压入栈（push）时，直接将元素压入目标队列。弹出栈（pop）时，将目标队列的元素都放入另外一个队列中，只留下一个元素。然后将这个元素弹出。同pop操作一样，top操作将目标队列的元素都放入另外一个队列中，只留下一个元素，然后将这个元素返回。pop操作和top操作注意目标队列的临界值。若目标队列为空，将缓存队列作为目标队列。代码如下：

class Stack {
public:
    // Push element x onto stack.
    void push(int x) {
        queueArr[currentQueue].push(x);
    }

    // Removes the element on top of the stack.
    void pop() {
        if(queueArr[currentQueue].empty()){
            currentQueue = (currentQueue + 1)%2;
        }
        while(queueArr[currentQueue].size()!=1){
            queueArr[(currentQueue + 1)%2].push(queueArr[currentQueue].front());
            queueArr[currentQueue].pop();
        }
        queueArr[currentQueue].pop();
    }

    // Get the top element.
    int top() {
        if(queueArr[currentQueue].empty()){
            currentQueue = (currentQueue + 1)%2;
        }
        while(queueArr[currentQueue].size()!=1){
            queueArr[(currentQueue + 1)%2].push(queueArr[currentQueue].front());
            queueArr[currentQueue].pop();
        }
        return queueArr[currentQueue].front();
    }

    // Return whether the stack is empty.
    bool empty() {
        return queueArr[0].empty() && queueArr[1].empty();
    }
    
    Stack(){
        currentQueue = 0;
    }
    
private:
    queue<int> queueArr[2];
    int currentQueue;
};

[LeetCode] Implement Stack using Queues

标签：c++   leetcode   

原文地址：http://blog.csdn.net/kangrydotnet/article/details/46492843