[每日 LeetCode] 232. Implement Queue using Stacks

原文链接 [每日 LeetCode] 232. Implement Queue using Stacks

Description:

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).

思路：本题要求用 stack 结构来实现队列的基本操作。由于队列的特点是先进先出，所以考虑使用两个 stack 来操作，一个 input 和一个 output，当 output 栈空时，从 input 栈中弹出元素压进 output 栈，如此一来即可实现后进的后出。

C++ 代码

class MyQueue {
    stack<int> input, output;
public:
    /** 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 temp = peek();
        output.pop();
        return temp;
    }
    /** 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();
    }
};
/**
 * Your MyQueue object will be instantiated and called as such:
 * MyQueue* obj = new MyQueue();
 * obj->push(x);
 * int param_2 = obj->pop();
 * int param_3 = obj->peek();
 * bool param_4 = obj->empty();
 */
​

运行时间：4ms

运行内存：9M