LeetCode 225. 用队列实现栈

题目描述：

解法一（两个队列，压入 -O(1)， 弹出 -O(n)）

class MyStack {
    queue<int> que1;
    queue<int> que2;
    int Top;
public:
    /** Initialize your data structure here. */
    MyStack() {
        
    }
    
    /** Push element x onto stack. */
    void push(int x) {
        que1.push(x);
        Top=x;
    }
    
    /** Removes the element on top of the stack and returns that element. */
    int pop() {
        int res=Top;
        while(que1.size()>1){
            Top=que1.front();
            que2.push(Top);
            que1.pop();
        }
        que1.pop();
        queue<int> temp=que1;
        que1=que2;
        que2=temp;
        return res;
    }
    
    /** Get the top element. */
    int top() {
        return Top;
    }
    
    /** Returns whether the stack is empty. */
    bool empty() {
        return que1.empty();
    }
};

/**
 * Your MyStack object will be instantiated and called as such:
 * MyStack* obj = new MyStack();
 * obj->push(x);
 * int param_2 = obj->pop();
 * int param_3 = obj->top();
 * bool param_4 = obj->empty();
 */

解法二（两个队列，压入 -O(n)， 弹出 -O(1)）：

class MyStack {
    queue<int> que1;
    queue<int> que2;
    int Top;
public:
    /** Initialize your data structure here. */
    MyStack() {
        
    }
    
    /** Push element x onto stack. */
    void push(int x) {
        Top=x;
        que2.push(x);
        while(!que1.empty()){
            que2.push(que1.front());
            que1.pop();
        }
        queue<int> temp=que1;
        que1=que2;
        que2=temp;
        
    }
    
    /** Removes the element on top of the stack and returns that element. */
    int pop() {
        if(!que1.empty()){
            int res=Top;
            que1.pop();
            Top=que1.front();
            return res;
        }
        else return -1;
    }
    
    /** Get the top element. */
    int top() {
        return Top;
    }
    
    /** Returns whether the stack is empty. */
    bool empty() {
        return que1.empty();
    }
};

/**
 * Your MyStack object will be instantiated and called as such:
 * MyStack* obj = new MyStack();
 * obj->push(x);
 * int param_2 = obj->pop();
 * int param_3 = obj->top();
 * bool param_4 = obj->empty();
 */

解法三（两个队列，压入 -O(n)， 弹出 -O(1)）：

class MyStack {
    queue<int> que;
    int Top;
public:
    /** Initialize your data structure here. */
    MyStack() {
        
    }
    
    /** Push element x onto stack. */
    void push(int x) {
        Top=x;
        int sz=que.size();
        que.push(x);
        while(sz--){
            que.push(que.front());
            que.pop();
        }
    }
    
    /** Removes the element on top of the stack and returns that element. */
    int pop() {
        if(!que.empty()){
            int res=Top;
            que.pop();
            Top=que.front();
            return res;
        }
        else return -1;
    }
    
    /** Get the top element. */
    int top() {
        return Top;
    }
    
    /** Returns whether the stack is empty. */
    bool empty() {
        return que.empty();
    }
};

/**
 * Your MyStack object will be instantiated and called as such:
 * MyStack* obj = new MyStack();
 * obj->push(x);
 * int param_2 = obj->pop();
 * int param_3 = obj->top();
 * bool param_4 = obj->empty();
 */