LeetCode Implement strStr()

Implement strStr().

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

public class Solution {
    public String strStr(String haystack, String needle) {
        if (haystack.equals(needle)) {
            return haystack;
        }        
        int haylen=haystack.length();
        int neelen=needle.length();
        if (neelen>haylen) {
            return null;
        }
        
        int i,j=0;
        int phay=0;
        while (phay<haylen-neelen) {
            i=phay;
            j=0;
            while (j<neelen && i<haylen && needle.charAt(j)==haystack.charAt(i)) {
                ++j;
                ++i;
            }
            if (j==neelen) {
                return haystack.substring(phay);
            }
            ++phay;
            
        }
        return null;
    }
}