155. Min Stack

Design a stack that supports push, pop, top, and retrieving the minimum element in constant time.

• push(x) -- Push element x onto stack.
• pop() -- Removes the element on top of the stack.
• top() -- Get the top element.
• getMin() -- Retrieve the minimum element in the stack.

 Example:MinStack minStack = new MinStack();minStack.push(-2);minStack.push(0);

minStack.push(-3); minStack.getMin(); --> Returns -3. minStack.pop(); minStack.top(); --> Returns 0. minStack.getMin(); --> Returns -2.
题目含义：设计一个最小栈，push, pop, top, 和 getMin 方法都是常量时间

方法一:使用辅助栈

首先需要一个正常栈normal,用于栈的正常操作，然后需要一个辅助栈minval，专门用于获取最小值，具体操作如下。

import java.util.Stack;

public class Solution {

    private Stack<Integer> values = new Stack<>();
    private Stack<Integer> miniStack = new Stack<>();
    
    public void push(int node) {
        values.push(node);
        if (miniStack.isEmpty()) miniStack.push(node);
        else miniStack.push(Math.min(miniStack.peek(), node));
    }

    public void pop() {
        if (values.isEmpty()) return;
        values.pop();
        miniStack.pop();
    }

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

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

方法二:

class MinStack {

   private Queue<Integer> p = new LinkedList<>();
    private Integer minValue = Integer.MAX_VALUE;

    /** Initialize your data structure here. */
    public MinStack() {
    }

    public int getMin() {

        for (int i=0;i<p.size();i++)
        {
            Integer value = p.poll();
            minValue = Math.min(minValue,value);
            p.add(value);
        }
        return minValue;
    }

    /** Push element x onto stack. */
    public void push(int x) {
        p.add(x);
        for (int i=1;i<p.size();i++)
        {
            p.add(p.poll());
        }
    }

    /** Removes the element on top of the stack and returns that element. */
    public int pop() {
        return p.poll();
    }

    /** Get the top element. */
    public int top() {
        return p.peek();
    }

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