leetcode- 232. Implement Queue using Stacks

使用栈实现队列：

将栈中的元素取出再存入一次就是队列：

class MyQueue {
public:
    /** Initialize your data structure here. */
    MyQueue() {
        
    }
    
    /** Push element x to the back of queue. */
    void push(int x) {
       s.push(x); 

    }
    
    /** Removes the element from in front of queue and returns that element. */
    int pop() {

        stack<int> s1;
        while(!s.empty())
        {
            int t=s.top();
            s.pop();
            s1.push(t);
            
        }
        int t=s1.top();
        s1.pop();
        while(!s1.empty())
        {
            int t=s1.top();
            
            s.push(t);
            s1.pop();
        }
        return t;
        
    }
    
    /** Get the front element. */
    int peek() {
        stack<int> s_tem=s;
        while(s_tem.size()>1)
        {
         s_tem.pop();
        }
        return s_tem.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();
 */

　　

更简单一点实现，pop和top的实现互相借助时：

class MyQueue {
public:
    stack<int> S1, S2;
    /** Initialize your data structure here. */
    MyQueue() {
           
    }
    
    /** Push element x to the back of queue. */
    void push(int x) {
        S1.push(x);
    }
    
    /** Removes the element from in front of queue and returns that element. */
    int pop() {
        int v = -1;
        if (!S2.empty()) { v = S2.top(); S2.pop();}
        else {
            while(!S1.empty()) {
                S2.push(S1.top());
                S1.pop();
            }
            v = S2.top();
            S2.pop();
        }
        return v;
    }
    
    /** Get the front element. */
    int peek() {
        if (!S2.empty()) return S2.top();
        else {
            while(!S1.empty()) {
                S2.push(S1.top());
                S1.pop();
            }
            return S2.top();
        }
    }
    
    /** Returns whether the queue is empty. */
    bool empty() {
        return (S1.empty()) && (S2.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();
 */