void Tempstr(char **str1,char **str2){
        char *temp;
        temp = *str1;*str1 = *str2;*str2 = temp;
    }//交换a，b，使小的为a
    int main(){
        
        int flag = 0;    
        char a1[20], b1[20];
        char *a = a1,*b = b1;
        scanf("%s", a1);
        scanf("%s", b1);
        if(strlen(a1) > strlen(b1))
            Tempstr(*a, *b);
        for(int i = 0, j = 0, z = 0; i <= strlen(b1) - strlen(a1); i++){
            //i是遍历b1，j是遍历a1，z是对比a1，b1的时候指向在b1的
            for(z = i,j = 0;j < strlen(a1) && a[j] == b[z]; j++){    
                z++;
            }
            if(j == strlen(a1)){
                printf("%s is substring of %s", a, b);
                flag = 1;
                break;    
            }
        }
      
        if(flag == 0)
            printf("No substring");
        
        
        return 0;
    }
···