剑指offer-20-包含min函数的栈

思路

• 空间换时间，辅助栈，存储最小值
  主------辅助
  5 ------ 2
  2 ------ 2
  4 ------ 3
  4 ------ 3
  3 ------ 3
  9 ------ 9

代码

import java.util.Stack;
public class Solution {
    Stack<Integer> stack=new Stack<>();
    Stack<Integer> help=new Stack<>();

    public void push(int node) {
        if(help.isEmpty()){  //help判断是否空
            help.push(node);
        }else{
            help.push(Math.min(node,help.peek()));
        }
        stack.push(node);
    }

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

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

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