Leetcode 232. Implement Queue using Stacks

原题：

 

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.

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).

 

解决方法：

 关键还是在push函数，弹出数直到stack为空。加入新的数，原来的数反序加入。

 

代码：

 

class MyQueue {
    stack<int> s;
public:
    /** Initialize your data structure here. */
    MyQueue() {
        
    }
    
    /** Push element x to the back of queue. */
    void push(int x) {
        if (s.size() == 0){
            s.push(x);
        }        
        else{
            int val = pop();
            push(x);
            s.push(val);
        }
    }
    
    /** Removes the element from in front of queue and returns that element. */
    int pop() {
        int val = s.top();
        s.pop();
        return val;
    }
    
    /** Get the front element. */
    int peek() {
        return s.top();
    }
    
    /** Returns whether the queue is empty. */
    bool empty() {
        return s.empty();
    }
};