笨死

思路：
1、第一个元素入栈
2、一致则出栈
3、不一致则继续入栈

import java.util.Stack;
public class Solution {
    public boolean IsPopOrder(int [] pushA,int [] popA) {
        if(pushA==null&&popA==null) return true;
        Stack<Integer> stack = new Stack<>();
        int i=0;
        int j=0;
        while(i<pushA.length){
            stack.push(pushA[i]);
            while(!stack.isEmpty()&&stack.peek()==popA[j]){
                //栈顶元素和当前popA一致就出栈
                stack.pop();
                j++;
            }
            //栈顶元素和当前popA不一致就继续入栈
            i++;
        }
        return stack.isEmpty();
    }
}