LeetCode 155. 最小栈

class MinStack {
    //定义一个数据栈s1，一个最小元素栈minStack
    Stack<Integer> s1,minStack;
    /** initialize your data structure here. */
    public MinStack() {
        s1 = new Stack<>();
        minStack = new Stack<>();
    }
    
    public void push(int x) {
        s1.push(x);
        if(minStack.isEmpty() || x <=  minStack.peek()){
            minStack.push(x);
        }
    }
    
    public void pop() {
        //不能用== ，用equals
        if(minStack.peek().equals(s1.peek())){
            minStack.pop();
        }
        s1.pop();
    }
    
    public int top() {
        return s1.peek();
    }
    
    public int getMin() {
        return minStack.peek();
    }
}

/**
 * Your MinStack object will be instantiated and called as such:
 * MinStack obj = new MinStack();
 * obj.push(x);
 * obj.pop();
 * int param_3 = obj.top();
 * int param_4 = obj.getMin();
 */