1.题目

2.画图分析

3.代码实现

package Stack;

import java.util.Stack;
public class MinStack {
    private Stack <Integer> stack;
    private Stack <Integer> MinStack;
    public MinStack() {
         stack = new Stack<>();
         MinStack = new Stack<>();
    }

    public void push(int val) {
        //普通栈只要是元素都得放
        //最小栈 如果为空,放元素.如果不为空且要放的元素比栈顶元素小,也要放
        stack.push(val);
        if(MinStack.empty()){
            MinStack.push(val);
        }
        else{
            int peek = MinStack.peek();
            if(val <= peek){
                MinStack.push(val);
            }
        }
    }

    public void pop() {
      int val = stack.pop();
      //如果最小栈不为空
        if(!MinStack.empty()){
            if(val == MinStack.peek()){
                MinStack.pop();
            }
        }
    }
    //获取普通栈的栈顶
    public int top() {
        return stack.peek();
    }

    public int getMin() {
        if(!MinStack.empty()){
                return MinStack.peek();
            }
        return -1;
    }
}