232 Implement Queue using Stacks

1 题目

Implement the following operations of a queue using stacks.

• push(x) -- Push element x to the back of queue.
• pop() -- Removes the element from in front of queue.
• peek() -- Get the front element.
• empty() -- Return whether the queue is empty.

Example:

MyQueue queue = new MyQueue();

queue.push(1);
queue.push(2);  
queue.peek();  // returns 1
queue.pop();   // returns 1
queue.empty(); // returns false

Notes:

• You must use only standard operations of a stack -- which means only push to top, peek/pop from top, size, and is empty operations are valid.
• Depending on your language, stack may not be supported natively. You may simulate a stack by using a list or deque (double-ended queue), as long as you use only standard operations of a stack.
• You may assume that all operations are valid (for example, no pop or peek operations will be called on an empty queue).

2 标准解

2.1 分析

 用两个栈input,output存储。输入时压入input栈顶，查看栈顶时，如果output为空，则将input元素逆序压入output中，此时output中的元素是FIFO的顺序，并且清空input。如果要删除元素，删除output栈顶元素。

2.2 代码

class MyQueue {
public:
    stack<int> input,output;
    /** Initialize your data structure here. */
    MyQueue() {
        
    }
    
    /** Push element x to the back of queue. */
    void push(int x) {
        input.push(x);
    }
    
    /** Removes the element from in front of queue and returns that element. */
    int pop() {
        int result = peek();
        output.pop();
        return result;
    }
    
    /** Get the front element. */
    int peek() {
        if(output.empty()){
            while(input.size()) {
                output.push(input.top());
                input.pop();
            }
        }
        return output.top();
    }
    
    /** Returns whether the queue is empty. */
    bool empty() {
        return input.empty()&&output.empty();
    }
};