LeetCode(64)- Min Stack

题目：

Design a stack that supports push, pop, top, and retrieving the minimum element in constant time.

push(x) -- Push element x onto stack.
pop() -- Removes the element on top of the stack.
top() -- Get the top element.
getMin() -- Retrieve the minimum element in the stack.

思路：

• 题意：给出四个函数API,构造一个stack，而且能够返回最小值
• 用双栈的策略，一个用来正常的存储，一个用来存贮最小值
• 注意比较的时候。peek（）函数要用equals函数比较，因为弹出的是对象

代码：

class MinStack {
    Stack<Integer> stack = new Stack<Integer>();
    Stack<Integer> min = new Stack<Integer>();
    public void push(int x) {
        if(min.isEmpty() || x <= min.peek()){
            min.push(x);
        }
        stack.push(x);
    }

    public void pop() {
        if(stack == null){
            return;
        }
        if(min.peek().equals(stack.peek())){
            min.pop();
        }
        stack.pop();
    }

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

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