Restore IP Address.java

public class Solution {  
    public ArrayList<String> restoreIpAddresses(String s) {  
        ArrayList<String> res = new ArrayList<String>();  
        if (s.length()<4||s.length()>12) return res;  
        dfs(s,"",res,0);  
        return res;  
    }  
      
    public void dfs(String s, String tmp, ArrayList<String> res, int count){  
        if (count == 3 && isValid(s)) {  
            res.add(tmp + s);  
            return;  
        }  
        for(int i=1; i<4 && i<s.length(); i++){  
            String substr = s.substring(0,i);  
            if (isValid(substr)){  
                dfs(s.substring(i), tmp + substr + '.', res, count+1);  
            }  
        }  
    }  
      
    public boolean isValid(String s){  
        if (s.charAt(0)=='0') return s.equals("0");  
        int num = Integer.parseInt(s);  
        return num<=255 && num>0;  
    }  
}