Leetcode daily question: implementing strstr()

link : Realization strStr()

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This problem is a bit like implementing functions strcmp() almost , But there is a difficult point , Is that if haystack There are multiple characters and... In needle The first character in is the same , And if the string after the first few same characters is the same as needle inequality , Then it needs to be judged again .
So the idea here is to use four pointers , Two of them are formal parameters of the function , So we're going to create two more pointers , Point to respectively haystack and needle.

	// If it is empty, it will directly return 0
    if (haystack == NULL)
        return 0;
    // Find the length of two strings 
    int haystacklen = strlen(haystack);
    int needlelen = strlen(needle);
    //needle Is longer than haystack, Then return directly -1
    if(haystacklen < needlelen)
        return -1;
    // Open up two pointers 
    char* ph = haystack;
    char* pn = needle;

Then there's the part that goes into the loop ：
Yes haystack The length of Cycle through , And then determine *ph Is it equal to needle First character of , If they are not equal, they are directly ph Take a step back .
If two characters are equal , Keep this first ph The location of , In case the two strings are inconsistent , The original location cannot be found when returning , therefore hold haystack Move to ph The location of . Then start Yes pn Length cycle of , If two ph and pn The characters in each shift are equal , Then exit the loop and return directly i Value , If they are not equal, they will ph Move to original haystack The location of , then pn go back to needle The location of , then break come out , Continue to cycle , Until the end of the length .

	int flag = 1;
    int i = 0;
    for(i = 0; i < haystacklen; i++)
    {
    
        if(*ph != *pn)
        {
    
            ph++;
        }
        else
        {
    
            haystack = ph;
            for(int j = 0; j < needlelen; j++)
            {
    
                if(*ph == *pn)
                {
    
                    ph++;
                    pn++;
                    flag = 0;
                }
                else
                {
    
                    ph = haystack;
                    ph++;
                    pn = needle;
                    flag = 1;
                    break;
                }
            }
            if(flag == 0)
                return i;
        }
    }
        return -1;

Complete code ：

int strStr(char * haystack, char * needle){
    
    if (haystack == NULL)
        return 0;
    // Find the length of two strings 
    int haystacklen = strlen(haystack);
    int needlelen = strlen(needle);
    //needle Is longer than haystack, Then return directly -1
    if(haystacklen < needlelen)
        return -1;
    // Open up two pointers 
    char* ph = haystack;
    char* pn = needle;
    
    int flag = 1;
    int i = 0;
    for(i = 0; i < haystacklen; i++)
    {
    
        if(*ph != *pn)
        {
    
            ph++;
        }
        else
        {
    
            haystack = ph;
            for(int j = 0; j < needlelen; j++)
            {
    
                if(*ph == *pn)
                {
    
                    ph++;
                    pn++;
                    flag = 0;
                }
                else
                {
    
                    ph = haystack;
                    ph++;
                    pn = needle;
                    flag = 1;
                    break;
                }
            }
            if(flag == 0)
                return i;
        }
    }
        return -1;
}