June training (day 13) - two way linked list

Preface

         This is a 《 Heroic algorithm Alliance ： Algorithm training 》 The content of , See details ： Knowledge of the planet ： Heroic algorithm Alliance - June training . After joining the planet , You can enjoy the star Lord CSDN Paid column Free reading The rights and interests of .
         Welcome to leave a message in the comment area to express your views , To know everything , can , Form the habit of brushing questions every day , You can also publish high-quality problem-solving reports by yourself , For community appreciation , Attract a wave of core fans .
         I hope you can think for yourself first , If you really have no idea , Let's look at the following algorithm ideas , If you have ideas but can't write them out , You can refer to the code of others in the circle of friends , There's always one for you , Keep an eye on him , Take its advantage , Short supply .
         The content of today's training is ： Double linked list
         There is only one question today , But it is a difficult problem , Be sure to work out this problem , If you still want to do this kind of problem , You can look at the may question .

One 、 Exercise questions