155. Min Stack

class MinStack {
    int min = Integer.MAX_VALUE;
    Stack<Integer> stack = new Stack<Integer>();
    /** initialize your data structure here. */
    public MinStack() {
        
    }
    
    public void push(int x) {
        if(x <= min){
            stack.push(min);
            min = x;
        }
        stack.push(x);
    }
    
    public void pop() {
        if(stack.pop() == min) min = stack.pop();
    }
    
    public int top() {
        return stack.peek();
    }
    
    public int getMin() {
        return min;
    }
}

232. Implement Queue using Stacks

用一个input和outlput栈，当pop时，output为空，则把input全部压入output中。

class MyQueue {
    Stack<Integer> input = new Stack();
    Stack<Integer> output = new Stack();
    /** Initialize your data structure here. */
    public MyQueue() {
        
    }
    
    /** Push element x to the back of queue. */
    public void push(int x) {
        input.push(x);
    }
    
    /** Removes the element from in front of queue and returns that element. */
    public int pop() {
        peek();
        return output.pop();
    }
    
    /** Get the front element. */
    public int peek() {
        if(output.empty())
            while(!input.empty())
                output.push(input.pop());
        return output.peek();
    }
    
    /** Returns whether the queue is empty. */
    public boolean empty() {
        return input.empty() && output.empty();
    }
}

225. Implement Stack using Queues

在push的时候就直接把顺序缓过来

class MyStack {
    Queue<Integer> queue;
    /** Initialize your data structure here. */
    public MyStack() {
        this.queue = new LinkedList<Integer>();
    }
    
    /** Push element x onto stack. */
    public void push(int x) {
        queue.add(x);
        for(int i = 0; i < queue.size() - 1; i++){
            queue.add(queue.poll());
        }
    }
    
    /** Removes the element on top of the stack and returns that element. */
    public int pop() {
        return queue.poll();
    }
    
    /** Get the top element. */
    public int top() {
        return queue.peek();
    }
    
    /** Returns whether the stack is empty. */
    public boolean empty() {
        return queue.isEmpty();
    }
}