题解 | #包含min函数的栈#

import java.util.Stack;

public class Solution {

    private Stack<Integer> stack1 = new Stack<>();
    private Stack<Integer> stack2 = new Stack<>();

    
    public void push(int node) {
        if (stack1.isEmpty()) {
            stack1.push(node);
            stack2.push(node);
        } else {
            int val = stack2.peek();
            if (val < node) {
                stack2.push(val);
            } else {
                stack2.push(node);
            }
            stack1.push(node);
        }
    }
    
    public void pop() {
        if (stack1.isEmpty()) {
            throw new RuntimeException("The stack is empty");
        }
        stack1.pop();
        stack2.pop();
    }
    
    public int top() {
        if (stack1.isEmpty()) {
            throw new RuntimeException("The stack is empty");
        }
        return stack1.peek();
    }
    
    public int min() {
        if (stack2.isEmpty()) {
            throw new RuntimeException("The stack is empty");
        }
        return stack2.peek();
    }
}