求解，一个C++的递归题目

Question

为什么答案是b?麻烦给个思考过程。

Answer 1

1. Recursively push the 2456 numbers one by one from the ones to the thousands into the stack

2. After the push is completed, the unstack begins. The unstack sequence is 2 4 5 6. During the unstack process, '1' is added to each bit, that is, 1 is added when displayed

3. The result is 3567

What should be examined is the understanding of the recursive stack