Generate IP Addresses

import java.util.*;

class Solution {
    public ArrayList<String> generateIp(String s) {
        ArrayList<String> result = new ArrayList<>();
        backtrack(s, 0, 0, "", result);
        return result;
    }

    private void backtrack(String s, int index, int parts, String current, ArrayList<String> result) {
        if (parts == 4 && index == s.length()) {
            result.add(current.substring(0, current.length() - 1));
            return;
        }

        if (parts >= 4)
            return;

        for (int len = 1; len <= 3 && index + len <= s.length(); len++) {

            String segment = s.substring(index, index + len);

            if (segment.length() > 1 && segment.startsWith("0"))
                continue;

            int value = Integer.parseInt(segment);

            if (value <= 255)
                backtrack(s, index + len, parts + 1, current + segment + ".", result);
        }
    }
}