kmp算法

#include <iostream>
#include <stdio.h>
#include <string>
using namespace std;
const int maxn=1000005;
char str[maxn],sub[maxn];
int  next[maxn];
void get_next(char *sub)//构造next 函数
{
     int i=1,j=0;
     next[1]=0;
     while(sub[i])
     {
          if(sub[i]==sub[j]||j==0)
          {
                ++i;++j;next[i]=j;
          }
          else j=next[j];
     }     
}
//判断sub是否是str的子串，匹配不成功返回-1,匹配成功返回第一次匹配的首位置
int kmp(char *str,char *sub)
{
     int i=1,j=1,ans=0;
     while(str[i])
     {
           if(str[i]==sub[j]||j==0)
           {
                i++;j++;
           }
           else j=next[j];

           if(sub[j]==0)
           {
                //ans++;j=next[j];//求有多少个子串
                return i-strlen(sub);                
           }
     }
     //return ans;
     return -1;
}

int main()
{
    int t,cc=0;
    cin>>t;
    while (t--)
    {
        scanf("%s%s",str,sub);
        get_next(sub);
        printf("Case %d: %d\n",++cc,kmp(str,sub));
    }
    return 0;
}