ImplementedStrStr.java

public class ImplementedStrStr {
 //With recursion:
 public int strStr(String haystack, String needle) {

 if (needle == null || needle == "") return 0;

 if (haystack == null) return -1;

 char[] cArr1 = haystack.toCharArray();
 char[] cArr2 = needle.toCharArray();
 for (int i = 0; i < cArr1.length; i++) {
 if (subString(cArr1, cArr2, i, 0)) return i;
 }
 return -1;
 }

 private boolean subString(char[] cArr1, char[] cArr2, int start, int index) {
 if (index == cArr2.length) {
 return true;
 }
 if (start + index >= cArr1.length || cArr1[start + index] != cArr2[index]) return false;
 return subString(cArr1, cArr2, start, index + 1);
 }

 public static void main(String[] args) {
 ImplementedStrStr s = new ImplementedStrStr();
 System.out.println(s.strStr("asd", "sd"));
 }
}