leetcode : Implement strStr()

Implement strStr().

Returns the index of the first occurrence of needle in haystack, or -1 if needle is not part of haystack.

tag : substring

public class Solution {
    public int strStr(String haystack, String needle) {
        if(haystack == null || needle == null || haystack.length() < needle.length()) {
            return -1;
        }
        int index = -1;
        for(int i = 0; i <= haystack.length() - needle.length(); i++) {
            int j;
            for(j = 0; j < needle.length(); j++) {
                if(needle.charAt(j) != haystack.charAt(i + j)) {
                    break;
                }
            }
            if(j == needle.length()) {
                return i;
            }
        }
        return index;
    }
}