[LC] 93. Restore IP Addresses

Given a string containing only digits, restore it by returning all possible valid IP address combinations.

Example:

Input: "25525511135"
Output: ["255.255.11.135", "255.255.111.35"]

class Solution {
    public List<String> restoreIpAddresses(String s) {
        List<String> res = new ArrayList<>();
        helper(res, 0, 0, s, "");
        return res;
    }

    private void helper(List<String> res, int count, int index, String s, String str) {
        if (count > 4) {
            return;
        }
        if (count == 4 && index == s.length()) {
            res.add(str);
        }
        for (int i = 1; i < 4; i++) {
            if (index + i > s.length()) {
                break;
            }
            String cur = s.substring(index, index + i);
            if (cur.charAt(0) == '0' && cur.length() > 1 || cur.length() == 3 && Integer.parseInt(cur) > 255) {
                continue;
            }
            String signal = count == 3 ? "": ".";
            helper(res, count + 1, index + i, s, str + cur + signal);
        }
    }

}