Implement strStr()

Implement strStr().

Returns a pointer to the first occurrence of needle in haystack, or null if needle is not part of haystack.

Solution:
We should consider it in C, since it mentioned pointer.
Returns a pointer in C is equal to return the rest of String in Java. 

// pointer???? --> substring of haystack

public class Solution {
    public String strStr(String haystack, String needle) {
       
        int m = haystack.length();
        int n = needle.length();
        
        if(n == 0)
            return haystack;
            
        for(int i = 0; i <= m - n; i++){
            if(haystack.charAt(i) == needle.charAt(0)){
                int j = i;
                int k = 0;
                while(j < m && k < n && haystack.charAt(j) == needle.charAt(k)){
                    j++;
                    k++;
                }
                if(k == n)
                    return haystack.substring(i);
            }
        }
        return null;
    }
}