155 最小栈

155 最小栈

题目：设计一个可以常数时间获取最小值的栈

示例：

输入：
["MinStack","push","push","push","getMin","pop","top","getMin"]
[[],[-2],[0],[-3],[],[],[],[]]

输出：
[null,null,null,null,-3,null,0,-2]

解释：
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.

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思考：

public class MinStack {
    /**
     * 栈1：数据栈，正常压入弹出
     */
    private LinkedList<Integer> stack1;

    /**
     * 栈2：辅助栈，栈顶保持栈1中的最小值
     */
    private LinkedList<Integer> stack2;

    public MinStack() {
        stack1 = new LinkedList<>();
        stack2 = new LinkedList<>();
    }

    public void push(int x) {
        // 栈2压入：1.x大就压入min；2.x小就压入x
        if (stack2.isEmpty() || x <= getMin()) {
            stack2.push(x);
        }
        if (x > getMin()) {
            stack2.push(getMin());
        }
        // 栈1每次正常进栈
        stack1.push(x);
    }

    public void pop() {
        if (stack1.isEmpty()) {
            throw new RuntimeException("MinStack is empty");
        }
        // 出栈是两个辅助栈都要出
        stack1.pop();
        stack2.pop();
    }

    public int top() {
        if (stack1.isEmpty()) {
            throw new RuntimeException("MinStack is empty");
        }
        return stack1.peek();
    }

    public int getMin() {
        if (stack2.isEmpty()) {
            throw new RuntimeException("MinStack is empty");
        }
        return stack2.peek();
    }
}

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