LeetCode --- 最小栈

设计一个支持 push，pop，top 操作，并能在常数时间内检索到最小元素的栈。

push(x) – 将元素 x 推入栈中。
pop() – 删除栈顶的元素。
top() – 获取栈顶元素。
getMin() – 检索栈中的最小元素。
示例:

MinStack minStack = new MinStack();
minStack.push(-2);
minStack.push(0);
minStack.push(-3);
minStack.getMin(); –> 返回 -3.
minStack.pop();
minStack.top(); –> 返回 0.
minStack.getMin(); –> 返回 -2.

public class MinStack {

    @Test
    public void test() {
        MinStack minStack = new MinStack();
        minStack.push(-2);
        minStack.push(0);
        minStack.push(-3);
        Assert.assertEquals(-3, minStack.getMin());
        minStack.pop();
        Assert.assertEquals(0, minStack.top());
        Assert.assertEquals(-2, minStack.getMin());
    }
    @Test
    public void test1() {
        MinStack minStack = new MinStack();
        minStack.push(512);
        minStack.push(-1024);
        minStack.push(-1024);
        minStack.push(512);
        minStack.pop();
        Assert.assertEquals(-1024, minStack.getMin());
        minStack.pop();
        Assert.assertEquals(-1024, minStack.top());
        minStack.pop();
        Assert.assertEquals(512, minStack.getMin());
    }

    //["MinStack","push","push","push","top","pop","getMin","pop","getMin","pop","push","top","getMin","push","top","getMin","pop","getMin"]
    //[[],[2147483646],[2147483646],[2147483647],[],[],[],[],[],[],[2147483647],[],[],[-2147483648],[],[],[],[]]
    //[null,null,null,null,2147483647,null,2147483646,null,2147483646,null,null,2147483647,2147483647,null,-2147483648,-2147483648,null,2147483647]
    @Test
    public void test2() {
        MinStack minStack = new MinStack();
        minStack.push(2147483646);
        minStack.push(2147483646);
        minStack.push(2147483647);
        Assert.assertEquals(2147483647, minStack.top());
        minStack.pop();
        Assert.assertEquals(2147483646, minStack.getMin());
        minStack.pop();
        Assert.assertEquals(2147483646, minStack.getMin());
        minStack.pop();
        minStack.push(2147483647);
        Assert.assertEquals(2147483647, minStack.top());
        Assert.assertEquals(2147483647, minStack.getMin());
        minStack.push(-2147483648);
        Assert.assertEquals(-2147483648, minStack.top());
        Assert.assertEquals(-2147483648, minStack.getMin());
        minStack.pop();
        Assert.assertEquals(2147483647, minStack.getMin());
    }

    Integer min;
    Integer elementData[];
    int elementCount = -1;

    /** initialize your data structure here. */
    public MinStack() {
        elementData = new Integer[100];
    }

    public void push(int x) {
        elementData[++elementCount] = x;
        if (min == null) {
            min = x;
        } else if (min > x) {
            min = x;
        }
    }

    public void pop() {
        if (elementCount > -1) {
            boolean flag = false;
            if (min.equals(elementData[elementCount])) {
                flag = true;
            }
            elementData[elementCount] = null;
            elementCount --;
            if (flag && elementCount > -2) {
                Optional min = Arrays.stream(elementData).filter(e -> null != e).min(Comparator.comparingInt(Integer::intValue));
                if (min.isPresent()) {
                    this.min = min.get();
                } else {
                    this.min = null;
                }
            }
        }
    }

    public int top() {
        return elementCount > -1 ? elementData[elementCount] : -1;
    }

    public int getMin() {
        return min;
    }

}