[C++] KMP算法

KMP算法

#include <iostream>
using namespace std;

void GetNext(const char* sub, int* next,int lensub)
{
  next[0] = -1;
  next[1] = 0;
  int i = 2;
  int k = 0;
  while(i < lensub)
  {
    if (k ==-1 || sub[i - 1] == sub[k])   //k==-1 意思是 前后缀没对应 使得 next[i] = 0
    {
      next[i] = k + 1;
      i++;
      k++;
    }
    else
    {
      k = next[k];  //找下标
    }
  }
  
}

int KMP(const char* str, const char* sub, int pos)
{
  int lenstr = strlen(str);
  int lensub = strlen(sub);
  if (lenstr == 0 || lensub == 0)
    return -1;
  if (pos < 0 || pos >= lenstr)
    return -1;

  int* next = (int *)malloc(sizeof(int)*lensub);
  GetNext(sub,next,lensub);

  int i = pos; //遍历主串
  int j = 0;  //遍历字串

  while (i < lenstr && j < lensub)
  {
    if (j== -1||str[i] == sub[j])
    {
      i++;
      j++;
    }
    else
      j = next[j];
  }
  if (j >= lensub)
    return i-j;
  return -1;
}

int main()
{
  cout << KMP("abcabcabcsabc","ac",0)<<endl;  //-1
  cout << KMP("adbacd","ac",0)<<endl;         //3
  return 0;
}