93. Restore IP Addresses

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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)

class Solution {    public ArrayList<String> restoreIpAddresses(String s) {        ArrayList<String> result = new ArrayList<String>();        ArrayList<String> list = new ArrayList<String>();        if(s.length() <4 || s.length() > 12)            return result;        helper(result, list, s , 0);        return result;    }    public void helper(ArrayList<String> result, ArrayList<String> list, String s, int start){        if(list.size() == 4){            if(start != s.length())                return;            StringBuffer sb = new StringBuffer();            for(String tmp: list){                sb.append(tmp);                sb.append(".");            }            sb.deleteCharAt(sb.length()-1);            result.add(sb.toString());            return;        }        for(int i=start; i<s.length() && i < start+3; i++){            String tmp = s.substring(start, i+1);            if(isvalid(tmp)){                list.add(tmp);                helper(result, list, s, i+1);                list.remove(list.size()-1);            }        }    }    private boolean isvalid(String s){        if(s.charAt(0) == '0')            return s.equals("0"); // to eliminate cases like "00", "10"        int digit = Integer.valueOf(s);        return digit >= 0 && digit <= 255;    }}

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