232 Implement Queue using Stacks
实现原理是使用两个stack,每当peek或者pop时将stack in中的元素全部转移到out。借此实现先入先出。

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

/**
 * 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();
 */

另一种实现方法是利用函数调用栈,每次插入元素时将待插入元素插到栈底。

class MyQueue {
public:
    /** Initialize your data structure here. */
    MyQueue() {
        
    }
    
    /** Push element x to the back of queue. */
    void push(int x) {
        push_helper(x);
    }
    
    void push_helper(int x){
        if(s.size() == 0)
        {
            s.push(x);
            return;
        }
        
        int val = s.top();
        s.pop();
        push_helper(x);
        s.push(val);
        return;
    }
    
    /** 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();
    }
private:
    stack<int> s;
};

/**
 * 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();
 */