Question:

Given two strings needle and haystack, return the index of the first occurrence of needle in haystack, or -1 if needle is not part of haystack.

Constraints:

1 <= haystack.length, needle.length <= 10 ⁴

Initial Thoughts

I can just use Java’s Inbuilt solution for this problem just do:

    public static int strStr(String haystack, String needle) {
        return haystack.indexOf(needle, 0);
    }

and to my surprise this worked 😂.
okay lets get back to the question. first the terminal conditions:

-> when needle > haystack
-> when needle is empty
-> when haystack is empty

    if (needle.length() > haystack.length() || needle.length() == 0 || haystack.length() == 0) return -1;

then coming to simple loop, this will remove the first character with each iteration until haystack becomes empty.

        int counter = 0;
        while (!haystack.equals("")) {
                //Code Here
            }
            counter++;
            haystack = haystack.substring(1);
        }

and finally add the logic where if the first characters match, check the other characters as well.

Initial Solution:

class Solution {
    public int strStr(String haystack, String needle) {
        // return haystack.indexOf(needle,0);
        if (needle.length() > haystack.length() || needle.length() == 0 || haystack.length() == 0) return -1;
        int counter = 0;
        while (!haystack.equals("")) {
            if (needle.charAt(0) == haystack.charAt(0)) {
                for (int i = 0; i < needle.length(); i++) {
                    if (haystack.length() - 1 < i) return -1;
                    if (needle.charAt(i) == haystack.charAt(i)) {
                        if (i == needle.length() - 1) {
                            return counter;
                        }
                    } else break;
                }
            }
            counter++;
            haystack = haystack.substring(1);
        }
        return -1;
    }
}

Peformance: