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

For example:
Given"25525511135",

return["255.255.11.135", "255.255.111.35"]. (Order does not matter)

tag: DFS

class Solution {
    public List<String> restoreIpAddresses(String s) {
        List<String> ans = new ArrayList<>();
        dfs(s, "", 0, 0, ans);

        return ans;
    }

    private void dfs(String s, String path, int index, int count, List<String> ans){
        if (count > 4) return;
        if (count == 4 && path.length() == s.length() + 3){
            ans.add(path);
        }

        for (int i = 1; i < 4; i++){
            if (index + i > s.length()) break;
            String cur = s.substring(index, index + i);
            if ( (cur.startsWith("0") && cur.length() > 1) || (cur.length() == 3 && Integer.valueOf(cur) >= 256) ) continue;
            dfs(s, path + cur + (count == 3 ? "" : "."), index + i, count + 1, ans);
        }
    }
}

public class Solution {
    public List<String> restoreIpAddresses(String s) {
        List<String> res = new ArrayList<String>();
        int len = s.length();
        for(int i = 1; i<4 && i<len-2; i++){
            for(int j = i+1; j<i+4 && j<len-1; j++){
                for(int k = j+1; k<j+4 && k<len; k++){
                    String s1 = s.substring(0,i), s2 = s.substring(i,j), s3 = s.substring(j,k), s4 = s.substring(k,len);
                    if(isValid(s1) && isValid(s2) && isValid(s3) && isValid(s4)){
                        res.add(s1+"."+s2+"."+s3+"."+s4);
                    }
                }
            }
        }
        return res;
    }
    public boolean isValid(String s){
        if(s.length()>3 || s.length()==0 || (s.charAt(0)=='0' && s.length()>1) || Integer.parseInt(s)>255)
            return false;
        return true;
    }
}