Simulate the implementation of strstr

 

strstr The arguments to the function are two char* Pointer to type , The return value is char* Pointer to type , Its function is to find whether the substring in the string exists , If there is, then   return str1 Neutron string （ originate str2） From , If it does not exist , Return null pointer

Let's implement mystrstr

1. First of all, design the parameter and the original function strstr equally
2. Assert first after entering the function p1 and p2 Null pointer or not
3. keep p2 Immobility , if p1 and p2 The content pointed to is different , be p1++
4. When p1 Content and p2 At the same time , use char*start Record p1 The address of
5. Redefine flag, Used to judge whether to enter while loop , Definition count Used to record p1 After entering the second cycle, compared with start Offset of position .
6. Under normal circumstances ,*p1==*p2,p1 and p2 own ++,count also ++
7. If you encounter the end of the substring you need to find , That is to say *p2==‘\0’ when flag=0. End of cycle , And back to start The address of
8. You may enter else sentence . The reason is that there are duplicate characters . Now start It is necessary to record the starting position again

 

 

char* mystrstr(const char* p1, const char* p2)
{
	assert(p1 && p2);
	while (*p1 != *p2)
	{
		p1++;
	}
	char* start = p1;
	int count = 0;
	int flag = 1;
	while (flag)
	{
		if (*p1 == *p2 )
		{
				p1++;
				p2++;
				count++;
		}
		else if (*p2 == '\0')
		{
			flag=0;
		}
		else
		{
			p2--;
			start++;
		}
	}
	return start;
}char* mystrstr(const char* p1, const char* p2)
{
	assert(p1 && p2);
	while (*p1 != *p2)
	{
		p1++;
	}
	char* start = p1;
	int count = 0;
	int flag = 1;
	while (flag)
	{
		if (*p1 == *p2 )
		{
				p1++;
				p2++;
				count++;
		}
		else if (*p2 == '\0')
		{
			flag=0;
		}
		else
		{
			p2--;
			start++;
		}
	}

Running results