Implement the strStr() function.
Given a haystack string and a needle string, find the first place (starting from 0) where the needle string appears in the haystack string. If not, returns - 1.
Example 1:
Input: haystack = "hello", needle = "ll"
Output: 2
Example 2:
Input: haystack = "aaaaa", needle = "bba"
Output: -1
Explain:
When need is an empty string, what value should we return? This is a good question in an interview.
For this problem, we should return 0 when need is an empty string. This matches the C language's strstr() and Java's indexOf() definitions.
Method 1:

class Solution {
public:
    int strStr(string haystack, string needle) {    
        if(needle.empty())
            return 0;
        if(haystack.length()<needle.length())
            return -1;
        int flag=1,p;
        for(int i=0;i<haystack.length()-needle.length()+1;i++)
        {
            p=i;
            for(int j=0;j<needle.length();j++)
            {
                if(haystack[p++]!=needle[j])//Mismatch
                {
                    flag=0;
                    break;
                }
                flag=1;//matching
            }
            if(flag==1)
                return i;
        }
        return -1;
    }
};

Method 2: use substr to get the substring with the specified length and position

class Solution {
public:
    int strStr(string haystack, string needle) {    
      if(needle == "") 
          return 0;        
      if(haystack.size() < needle.size())
            return -1;      
       int p;       
       for(int i = 0;i < haystack.size()-needle.size()+1;++i)
       {       
            if(haystack.substr(i,needle.size()) == needle)              
                return i;       
        }      
        return -1; 
    }
};

Knowledge supplement:
Do this problem to know that c + + and strstr function, used to find the location of the first occurrence of a string, return a pointer.
Archetype:

    const char * strstr ( const char * str1, const char * str2 );
         		char * strstr (       char * str1, const char * str2 );

Usage:

#include <iostream>
#include <string.h>
using namespace std;
int main()
{
	char *s="1234abcd";
	char *sb1="34a"; 
	int p;
	p=strstr(s,sb1)-s;
	cout<<p;
	return 0;
}

Output 2