（1）暴力旋轉O(N^2)

#include<stdio.h>
#include<string.h>

void rotate_str(char *str,int k)
{
  int len = strlen(str);
  while(k%len--)
  {
    char ch = str[0];
    char *p1 =str;
    char *p2 = str+1;
    while(*p2)
    {
      *p1 = *p2;
      p2++;
      p1++;
    }
    *p1 = ch;
  }
}

int main()
{
  char str[] = "ABCDEF";
  int k = 12;
  printf("%s\n",str);
  rotate_str(str,k);
  printf("%s\n",str);
  return 0;
}
 

（2）要左旋k個字元，先把前k個字元逆置，再把剩餘的字元逆置，最後整體逆置

#include<stdio.h>
#include<string.h>
//先逆置k個字元，再把剩餘字元逆置，最後整體逆置

void Swap(char *p1,char *p2)
{
  char tmp = *p1;
  *p1 = *p2;
  *p2 = tmp;
}

void rotate(char *str,int k)
{
  int len = strlen(str);
  char *p1 = str;
  char *p2 =str + k%len-1;
  char *p3 = p2+1;
  char *p4 = str+len-1;
  while(p1<=p2)
  {
    Swap(p1,p2);
    p1++;
    p2--;
  }

  while(p3<=p4)
  {
    Swap(p3,p4);
    p3++;
    p4--;
  }

  p1 = str;
  p4 = str+len-1;

  while(p1<=p4)
  {
    Swap(p1,p4);
    p1++;
    p4--;
  }
  
}


int main()
{
  char str[] = "1234abcdef";
  int k = 21;
  printf("%s\n",str);
  rotate(str,k);
  printf("%s\n",str);
  return 0;
}
 

（3）把兩個相同字符合並，建立雙字串，需要旋轉k則從第k個開始取length個

//在一個字串後面再跟一個相同字串（建立雙倍字串）
#include<stdio.h>
#include<string.h>
#include<malloc.h>

void rotate(char *msg,int len,int k)
{
  int step = k%len;
  char *newStr = (char *)malloc(2*len+1);
  strcpy(newStr,msg);
  strcat(newStr,msg);
  strncpy(msg,newStr+step,len);
  free(newStr);
}


int main()
{
  char msg[] = "1234abcdef";
  int len = strlen(msg);
  int k = 3;
  printf("%s\n",msg);
  rotate(msg,len,k);
  printf("%s\n",msg);
  return 0;
}