155. Min Stack / Min Stack

leet和lint不太一样，leet多一个top()。但实现是一样的。

minStack只能用两个stack实现，不能用别的。

public class MinStack {
    private Stack<Integer> stack;
    private Stack<Integer> minStack;

    /** initialize your data structure here. */
    public MinStack() {
        stack = new Stack<>();
        minStack = new Stack<>();
    }

    public void push(int x) {
        stack.push(x);
        if (minStack.isEmpty()) {
            minStack.push(x);
        } else minStack.push(Math.min(x, minStack.peek()));
    }

    public void pop() {
        minStack.pop();
        stack.pop();
    }

    public int top() {
        return stack.peek();
    }

    public int getMin() {
        return minStack.peek();
    }
}

在上面这种实现中，我们实际上push到minStack中很多冗余的元素，下面的解法可以节省一定的空间，但两者的空间复杂度是相同的。

public class MinStack {
    private Stack<Integer> stack;
    private Stack<Integer> minStack;

    public MinStack() {
        stack = new Stack<Integer>();
        minStack = new Stack<Integer>();
    }

    public void push(int number) {
        stack.push(number);
        if (minStack.empty() == true)
            minStack.push(number);
        else {
        // 这里考虑的相等的情况也会继续push
        if (minStack.peek() >= number)
            minStack.push(number);
        }
    }

    public int pop() {
        if (stack.peek().equals(minStack.peek()) ) 
            minStack.pop();
        return stack.pop();
    }

    public int min() {
        return minStack.peek();
    }
}

232. Implement Queue using Stacks

年少无知时第一次做这题，拿两个stack底部相接，妄图拼出一个queue来。现在想起来简直笑出声。

public class MyQueue {
    private Stack<Integer> stack1;
    private Stack<Integer> stack2;
    /** Initialize your data structure here. */
    public MyQueue() {
        stack1 = new Stack<>();
        stack2 = new Stack<>();
    }

    private void stack2To1() {
        while (!stack2.isEmpty()) {
            stack1.push(stack2.pop());
        }
    }

    /** Push element x to the back of queue. */
    public void push(int x) {
        stack2.push(x);
    }

    /** Removes the element from in front of queue and returns that element. */
    public int pop() {
        if (stack1.isEmpty()) {
            this.stack2To1();
        }
        return stack1.pop();
    }

    /** Get the front element. */
    public int peek() {
        if (stack1.isEmpty()) {
            this.stack2To1();
        }
        return stack1.peek();
    }

    /** Returns whether the queue is empty. */
    public boolean empty() {
        return (stack1.isEmpty() && stack2.isEmpty());
    }
}

225. Implement Stack using Queues

public class MyStack {
    private Queue<Integer> queue1;
    private Queue<Integer> queue2;
    /** Initialize your data structure here. */
    public MyStack() {
        queue1 = new LinkedList<>();
        queue2 = new LinkedList<>();
    }

    private void move() {
        while (queue1.size() != 1) {
            queue2.offer(queue1.poll());
        }
    }

    private void swap() {
        Queue<Integer> tmp = queue1;
        queue1 = queue2;
        queue2 = tmp;
    }

    /** Push element x onto stack. */
    public void push(int x) {
        queue1.offer(x);
    }

    /** Removes the element on top of the stack and returns that element. */
    public int pop() {
        move();
        int item = queue1.poll();
        swap();
        return item;
    }

    /** Get the top element. */
    public int top() {
        move();
        int item = queue1.poll();
        swap();
        queue1.offer(item);
        return item;
    }

    /** Returns whether the stack is empty. */
    public boolean empty() {
        return queue1.isEmpty();
    }
}