构建一个栈，使其有查找最小元素的功能

数据栈+辅助栈

class MinStack {

    /** initialize your data structure here. */
    private Stack<Integer> dataStack;
    private Stack<Integer> minStack;
    private int min;
    public MinStack() {
        dataStack=new Stack<>();
        minStack=new Stack<>();
        min=Integer.MAX_VALUE;
    }
    
    public void push(int x) {
        dataStack.add(x);     
        min=Math.min(min,x); 
        minStack.add(min);
    }
    
    public void pop() {
        dataStack.pop();
        minStack.pop();
        min=minStack.isEmpty()?Integer.MAX_VALUE:minStack.peek();
    }
    
    public int top() {
        return dataStack.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();
 */