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,s,0,0,"");
        
        return res;
    }
    
    public void helper(List<String> res, String ip, int index, int count,String store)     {
         if(count > 4) return;
        
        if(count == 4 && index == ip.length()) res.add(store) ;
        
        
        for(int i = 1 ; i < 4; i++){
            if(index + i > ip.length()) break;
            
            String s = ip.substring(index,index+i);
            
            if(s.startsWith("0") && s.length() > 1 || i ==3 && Integer.parseInt(s) >=256) continue;
            
            
            helper(res,ip,index+i,count+1,store+s+(count == 3 ? "":"."));
        }
    }
}