实现一个特殊的栈，要求push,poll , getMin方法时间复杂度都是O（N）

借助两个栈来实现

public class GetMinStack {
 
    private Stack<Integer> stackData;
    private Stack<Integer> stackMin;
 
    public GetMinStack() {
        this.stackData = new Stack<Integer>();
        this.stackMin = new Stack<Integer>();
    }
 
    public void push(int obj) {
        if (stackMin.isEmpty()) {
            stackMin.push(obj);
        } else {
            stackMin.push(obj < getmin() ? obj : getmin());
        }
        stackData.push(obj);
    }
 
    public int getmin() {
        if (stackMin.isEmpty()) {
            throw new RuntimeException("Your stack is empty.");
        }
        return stackMin.peek();
    }
 
    public int pop() {
        if (this.stackData.isEmpty()) {
            throw new RuntimeException("Your stack is empty.");
        }
        this.stackMin.pop();
        return this.stackData.pop();
    }
 
}