Reverse placement of a single-chain table.

The reason for the code order in the reverse function to change the result is clear.

If you do not change the head address first, the address of mid-> link changes the address of head-> link, and the result is incorrect.

Head = head-> link; // this line
Mid-> link = last; // if this row exists, the result is incorrect. Why is it wrong?

 

 

 

 

# Include <stdio. h> <br/> # include <stdlib. h> <br/> typedef struct test <br/>{< br/> int num; <br/> struct test * link; <br/>} Node; <br/> Node * insert (Node * head, int num) <br/> {<br/> Node * new_node, * ptr = head; <br/> new_node = (Node *) malloc (sizeof (Node); <br/> if (! New_node) <br/>{< br/> printf ("error/n"); <br/> exit (1 ); <br/>}< br/> new_node-> num = num; <br/> new_node-> link = NULL; <br/> if (! Head) <br/>{< br/> return new_node; <br/>}< br/> while (ptr-> link! = NULL) <br/>{< br/> ptr = ptr-> link; <br/>}</p> <p> ptr-> link = new_node; <br/> return head; <br/>}< br/> Node * reverse (Node * head) <br/>{< br/> Node * last, * mid = NULL; <br/> while (head! = NULL) <br/>{< br/> last = mid; <br/> mid = head; // here the mid and head point to the same node <br/> head = head-> link; // update the head first, head and mid point to different nodes <br/> mid-> link = last; // the link of the mid is changed here. If the head is not updated before, the link of the head is changed. <br/>}< br/> return mid; <br/>}< br/> void show (Node * head) <br/> {<br/> Node * temp = head; <br/> while (temp! = NULL) <br/>{< br/> printf ("% d->", temp-> num); <br/> temp = temp-> link; <br/>}< br/> printf ("NULL/n"); <br/>}< br/> void frees (Node * head) <br/>{< br/> while (head! = NULL) <br/>{< br/> head = head-> link; <br/> free (head ); <br/>}< br/> int main () <br/>{< br/> Node * head = NULL; <br/> int n, m, I; <br/> printf ("How many numbers do I need to enter? N =: "); <br/> scanf (" % d ", & n); <br/> for (I = 1; I <= n; I ++) <br/>{< br/> printf ("Enter the % d Number:", I); <br/> scanf ("% d", & m ); <br/> head = insert (head, m); <br/>}< br/> head = reverse (head); <br/> printf ("result: "); <br/> show (head); <br/> frees (head); <br/> return 0; <br/>}