题目思路:
使用一个栈来进行入栈,然后当栈不为空则开始跟popA数组进行比较,若相等则将该元素弹出栈
最后看栈是否为空,如果为空则证明popA是pushA的出栈数组。

 public boolean IsPopOrder(int [] pushA,int [] popA) {
        if(pushA.length == 0 && popA.length == 0)
          return true;
        Stack<Integer> stack = new Stack<>();
        int n = pushA.length;
        for(int pushIndex = 0,popIndex = 0;pushIndex < n; pushIndex++){
            stack.push(pushA[pushIndex]);
            while(popIndex < n && !stack.isEmpty() && stack.peek()==popA[popIndex]){
                stack.pop();
                popIndex++;
            }
        }
        return stack.isEmpty();
    }