题目描述

定义栈的数据结构，请在该类型中实现一个能够得到栈中所含最小元素的min函数（时间复杂度应为O（1））

题目链接：https://www.nowcoder.com/practice/4c776177d2c04c2494f2555c9fcc1e49

public class Solution {

    private int size;
    private int min = Integer.MAX_VALUE;
    private Stack<Integer> stack = new Stack<>();
    private Integer[] elements = new Integer[10];
    
    public void push(int node) {
        ensureCapacity(size+1);
        elements[size++] = node;
        if (node < min) {
            stack.push(node);
            min = node;
        } else {
            stack.push(min);
        }
    }
    
    private void ensureCapacity(int size) {
        int len = elements.length;
        if (size > len) {
            int newLen = (len * 3) / 2 + 1;
            elements = Arrays.copyOf(elements, newLen);
        }
    }
    
    public void pop() {
        Integer top = top();
        if (top != null) {
            elements[size-1] = null;
            size--;
            stack.pop();
            min = stack.peek();
        }
    }
    
    public int top() {
        if (size != 0) {
            if (size - 1 >= 0) {
                return elements[size-1];
            }
        }
        return (Integer) null;
    }
    
    public int min() {
        return min;
    }
}