C语言模拟实现strstr函数的示例代码

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

int main(){

    char* s1 = "abcdefgh";
    char* s2 = "def";
    char* s3 = "dee";
    
    printf("%s\n",strstr(s1,s2)); //defgh   
    printf("%s\n",strstr(s1,s3)); //(null)

    return 0;
}

char* my_strstr(char* str1, char* str2){
    assert(str1 && str2);
    
    int slen = strlen(str1);
    int sublen = strlen(str2);
    
    int i = 0;
    int j = 0;
    int count = 0;

    while(i < slen){
        
        while(str1[i] == str2[j] &&  j < sublen){
            ++i;
            ++j;
        }

        if(j >= sublen){
            return str1 + i - j;
        }
        
        ++count;
        i = count;
        j = 0;
        
    }        

    return NULL;

}

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

void get_next(int* next, char* sub){
    int len = strlen(sub);
    next[0] = -1;
    next[1] = 0;

    int i = 2;
    int k = 0;

    while(i < len){
        if(k == -1 || sub[i-1] == sub[k]){
            next[i] = ++k;
            ++i;
        }else{
            k = next[k];
        }
    }
    
    
    
}

char* my_strstr(char *str1, char * str2){
    assert(str1 && str2);
    
    int slen = strlen(str1);
    int sublen = strlen(str2);

    int* next = (int*)malloc(sizeof(int)*sublen);
    assert(next);
    get_next(next,str2);

    int i = 0;
    int j = 0;

    while(i < slen && j < sublen){
        if(j == -1 || str1[i] == str2[j]){
            ++i;
            ++j;
        }else{
            j = next[j];
        }
    }

    if(i >= sublen){
        return str1 + i - j;
    }else{
        return NULL;
    }
    
}