See how http://pythontutor.com/c.html visualizes structures and pointers.

tryPythonTutorCPointers.c

// for http://pythontutor.com/c.html#mode=edit
// forfor https://adnmb.com/t/18445046
#include<stdlib.h> // NULL

typedef struct account{
  int num;
  struct account *next;
} Account;
typedef Account* APtr;

APtr newA(int num_){
  APtr n = (APtr)malloc(sizeof(Account));
  n->num = num_;
  n->next = NULL;
  return n;
}

int main() {
  // 新建两个结构体
  APtr alice=newA(42);
  APtr bob=newA(9);

  // 两个结构体的 next 互相指向对方
  alice->next = bob;
  bob->next = alice;

  // 利用临时变量 eve 来交换 alice 和 bob 两个指针所指的结构体
  APtr eve = alice;
  alice = bob;
  bob = eve;

  // 试一下 free 释放内存的可视化效果
  free(bob);
  free(alice);
  return 0;
}