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LeetCode Solution - Restore IP Addresses

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https://leetcode.com/problems/restore-ip-addresses/

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)

Keywords: ALL POSSIBLE gives us hint to use DFS

Another important point is that we should eliminate the case like '00', '01', '001', etc.

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public class Solution {
    /**
    * @param s the IP string
    * @return All possible valid IP addresses
    */
    public ArrayList<String> restoreIpAddresses(String s) {
        // Write your code here
        char[] str = s.toCharArray();
        int pointNums = 0;
        ArrayList<String> result = new ArrayList<String>();
        String temp = new String();
    
        if (s == null || s.length() < 4 || s.length() > 12) {
            return result;
        }
        restoreIpAddressesHelper(result, temp, str, 0, pointNums);
        return result;
    }

    private void restoreIpAddressesHelper(ArrayList<String> result,
                                            String temp,
                                            char[] str,
                                            int pos,
                                            int pointNums) {
        if (temp.length() == str.length + 3 && pointNums == 3) {
            result.add(temp);
        }
        if (pointNums > 3) {
            return;
        }
    
        StringBuffer addString = new StringBuffer();
        for (int i = pos; i < pos + 3 && i < str.length; i++) {
            addString.append(str[i]);
            if (!isValidHelper(addString.toString())) {
                break;
            }
            if ( i == str.length - 1) {
                restoreIpAddressesHelper(result, temp + addString, 
                                            str, i + 1, pointNums);
            } else {
                restoreIpAddressesHelper(result, temp + addString + ".", 
                                            str, i + 1, pointNums + 1);
            }
        }
        return;     
    }
    private boolean isValidHelper(String s) {
        if (s.charAt(0) == '0' && s.length() != 1) {
            return false;
        } else if (Integer.valueOf(s) > 255 ) {
            return false;
        } else {
            return true;
        }
    }
}