#include <stdio.h> 
#include <malloc.h>  
#include <stdlib.h> 
 
/************** start of stack *************/ 
 
#define STACK_SIZE 1024 
 
char stack[STACK_SIZE]; 
int top = 0; 
 
void push(char ch){ 
  stack[top] = ch; 
  top++; 
} 
 
char pop(){ 
  top--; 
  return stack[top]; 
} 
 
int isempty(){ 
  return 0 == top; 
} 
 
void test_stack(){ 
  push('a'); 
  push('b'); 
  push('c'); 
  push('d'); 
 
  while(!isempty()){ 
    printf("pop ch: %c\n", pop()); 
  } 
} 
 
/************** end of stack *************/ 
 
 
 
 
struct _node{ 
  char data; 
  struct _node *next; 
}; 
 
typedef struct _node node, *plink; 
 
 
plink init_link(){ 
  plink pl; 
  pl = (plink)malloc(sizeof(node)); 
   
  // check malloc success or not 
  if(NULL == pl) { 
    printf("malloc memory fail..."); 
    return NULL; 
  } 
   
  // init link head 
  pl->data = '\0'; 
  pl->next = NULL; 
   
  return pl; 
} 
 
void input_data(plink pl, char data){ 
  plink p = pl; 
   
  while(p->next){ 
    p = p->next; 
  } 
   
  plink node = NULL; 
  node = (plink)malloc(sizeof(node));   // malloc a new node 
   
  // add data 
  if(NULL != node){ 
    node->data = data; 
    node->next = p->next;    // last next is NULL 
    p->next = node; 
    p = node;          // p point last node 
  } 
} 
 
void output_link(plink pl){ 
  if(NULL == pl){ 
    printf("plink is null"); 
    return; 
  } 
   
  plink p = pl->next;  // already check pl is NULL, so here is ok 
  while(NULL != p){ 
    printf("%c -> ", p->data); 
    p = p->next; 
  }  
  printf("\n\n");     
} 
 
 
 
// push and pop stack 
plink revert_link2(plink pl){ 
  plink p = pl; 
 
  while(p->next){ 
//    printf("p->data: %c\n", p->next->data); 
    if(p->next->next){ 
      push(p->next->next->data); 
      push(p->next->data); 
      p = p->next->next; 
    } else { 
      push(p->next->data); 
      p = p->next; 
    } 
  } 
 
  while(!isempty()){ 
    printf("%c -> ", pop()); 
  } 
   
  printf("\n\n"); 
 
  return NULL; 
} 
 
 
 
plink revert_link(plink pl){ 
  if(NULL == pl){  // check link is NULL 
    return NULL; 
  } 
   
  int link_len = 0; 
  plink tmp_pl = pl->next; 
   
  while(tmp_pl){  // count link count 
    link_len++; 
    tmp_pl = tmp_pl->next; 
  } 
   
  // link length is no more than two node(s) 
  if(link_len <= 2){ 
    return pl; 
  } 
   
  // link length is more than two nodes 
  return revert_link2(pl); 
} 
 
 
 
 
int main(){ 
  plink pl = NULL; 
   
  pl = init_link();     // init link head 
   
  input_data(pl, 'a');   // add data 
  input_data(pl, 'b'); 
  input_data(pl, 'c'); 
  input_data(pl, 'd'); 
  input_data(pl, 'e'); 
  input_data(pl, 'f'); 
  input_data(pl, 'g'); 
   
  output_link(pl); 
   
  plink pl2 = revert_link(pl); 
   
  output_link(pl2); 
 
  return 0; 
} 
 
 
 
/**** 
revert_link.c 
 
 
linux gcc compile 
gcc revert_link.c -o revert_link && ./revert_link 
 
 
output result: 
 
a -> b -> c -> d -> e -> f -> g 
g -> e -> f -> c -> d -> a -> b 
 
or 
 
a -> b -> c -> d -> e -> f 
e -> f -> c -> d -> a -> b 
 
 
****/ 

plink revert_link3(plink pl){ 
  if(NULL == pl){ 
    printf("plink is null"); 
    return NULL;   
  }   
 
  plink p = pl; 
  plink first = p->next; 
  while(NULL != first){ 
    plink second = first->next; 
    if(NULL != second){ 
      first->next = second->next;   // third node 
      second->next = first;      // revert two nodes 
      first = first->next; 
      p->next = second; 
      p = second->next; 
    } 
  } 
  return pl; 
}