#include "stdio.h"
#include "stdlib.h"
 
void reverse_str(char str[],int n,int m);
 
int main(void)
{
  char str[] = "abcdef";
  reverse_str(str,6,2);
  return 0;
}
 
//str为字符串数组，n为数组长度，m为左移位数
void reverse_str(char str[],int n,int m)
{
  int i,j;
  char tmp; 
     
  for(i=0,j=n-1;i<j;i++,j--)
  {
    tmp = str[i];
    str[i] = str[j];
    str[j] = tmp;
  }
 
  for(i=0,j=n-m-1;i<j;i++,j--)
  {
    tmp = str[i];
    str[i] = str[j];
    str[j] = tmp;
  }
 
  for(i=n-m,j=n-1;i<j;i++,j--)
  {
    tmp = str[i];
    str[i] = str[j];
    str[j] = tmp;
  }
 
  printf("%s\n",str);
}

#include "stdio.h"
#include "stdlib.h"
 
void helper(char a[],int n);
 
int main(void)
{
  char str[15] = "I am a student!";
  helper(str,15);
  printf("\n");
  return 0;
}
 
void helper(char a[],int n)
{
  int e = n-1;
  int i,j,t;
   
  for(i=e;i>=0;i=j-1)
  {
    for(j=i;j>=0 && a[j]!=' ' ;--j);
    t=j+1;
    while(t<=i)
      printf("%c",a[t++]);
    if(j<0)
      return;
    else  
      printf(" ");
  }
}