！！！，all right，thx

the second example of B: 3 1 2. why it is NO？we can move it like this.

-------------------------1------------------1---------------1-------------------------------2
3-1-2 ——> 3-null-2 ——> null-3-2 ——> null-3-2 ——> 1-3-2 ——> 1-3-null ——> finished and yes