leetcode 155. Min Stack 、232. Implement Queue using Stacks 、225. Implement Stack using Queues

155. Min Stack

class MinStack {
public:
    /** initialize your data structure here. */
    MinStack() {
    }
    
    void push(int x) {
        if(s1.empty() && s2.empty()){
            s1.push(x);
            s2.push(x);
        }
        else{
            if(x < s2.top()){
                s1.push(x);
                s2.push(x);
            }
            else{
                s1.push(x);
                s2.push(s2.top());
            }
        }
    }
    
    void pop() {
        s1.pop();
        s2.pop();
        return;
    }
    
    int top() {
        return s1.top();
    }
    
    int getMin() {
        return s2.top();
    }
    stack<int> s1,s2;
};

 

 

232. Implement Queue using Stacks

class MyQueue {
public:
    /** 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() {
        if(s2.empty())
            shiftstack();
        int tmp = s2.top();
        s2.pop();
        return tmp;
    }
    
    /** Get the front element. */
    int peek() {
        if(s2.empty())
            shiftstack();
        return s2.top();
    }
    
    /** Returns whether the queue is empty. */
    bool empty() {
        return s1.empty() && s2.empty() ? true : false;
    }
    
    void shiftstack(){
        if(s1.empty())
            return;
        while(!s1.empty()){
            int tmp = s1.top();
            s1.pop();
            s2.push(tmp);
        }
        return;
    }
        
    stack<int> s1,s2;
};

 

225. Implement Stack using Queues

将存储的队列之前的数值再次加入到队列的末尾

class MyStack {
public:
    /** Initialize your data structure here. */
    MyStack() {
        
    }
    
    /** Push element x onto stack. */
    void push(int x) {
        q.push(x);
        for(int i = 0;i < q.size() - 1;i++){
            int tmp = q.front();
            q.pop();
            q.push(tmp);
        }
        return;
    }
    
    /** Removes the element on top of the stack and returns that element. */
    int pop() {
        int tmp = q.front();
        q.pop();
        return tmp;
    }
    
    /** Get the top element. */
    int top() {
        return q.front();
    }
    
    /** Returns whether the stack is empty. */
    bool empty() {
        return q.empty();
    }
    queue<int> q;
};