最小栈
最小栈

class MinStack {
    private Stack<Integer> stack;
    private Stack<Integer> minstack;
    public MinStack() {
        stack=new Stack<>();
        minstack=new Stack<>();
    }
    
    public void push(int val) {
        stack.push(val);
        if(minstack.empty()){
            minstack.push(val);
        }else{  //=要加上，否则stack中此数有多个 出栈时minstack也要出栈 数不够只有一个
            if(val<=minstack.peek()){
                minstack.push(val);
            }
        }
    }
   
    public void pop() {
        if(!stack.empty()){
            int ret=stack.pop();
            if(minstack.peek()==ret){
                minstack.pop();
            }
        }
    }
     //获取正常栈顶元素
    public int top() {
        if(stack.empty()){
            return -1; //oj题中轻易不要抛出异常 直接return-1即可
        }
        return stack.peek();
    }
    //获取最小栈顶元素
    public int getMin() {
        if(minstack.empty()){
            return -1;
        }
        return minstack.peek();
    }
}