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:

 1 class MinStack {
 2 
 3    private Queue<Integer> p = new LinkedList<>();
 4     private Integer minValue = Integer.MAX_VALUE;
 5 
 6     /** Initialize your data structure here. */
 7     public MinStack() {
 8     }
 9 
10     public int getMin() {
11 
12         for (int i=0;i<p.size();i++)
13         {
14             Integer value = p.poll();
15             minValue = Math.min(minValue,value);
16             p.add(value);
17         }
18         return minValue;
19     }
20 
21     /** Push element x onto stack. */
22     public void push(int x) {
23         p.add(x);
24         for (int i=1;i<p.size();i++)
25         {
26             p.add(p.poll());
27         }
28     }
29 
30     /** Removes the element on top of the stack and returns that element. */
31     public int pop() {
32         return p.poll();
33     }
34 
35     /** Get the top element. */
36     public int top() {
37         return p.peek();
38     }
39 
40     /** Returns whether the stack is empty. */
41     public boolean empty() {
42         return p.isEmpty();
43     }
44 }