Restore IP Addresses

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

Example

Given "25525511135", return

[
  "255.255.11.135",
  "255.255.111.35"
]

Order does not matter.

这题和排列题一样。先取IP中的第一个部分，然后递归得到后面IP部分，最后把他们合起来。但是里面有几个需要注意的test case。

public class Solution {
    public List<String> restoreIpAddresses(String s) {
        List<String> list = new ArrayList<>();
        helper(list, s, "", 3);
        return list;
    }
    private void helper(List<String> result, String str, String temp, int dotsLeft) {
        if (dotsLeft == -1 && str.length() == 0) {
            result.add(temp);
            return;
        }
        if (dotsLeft <= -1) return;
        
        for (int i = 1; i <= Math.min(str.length(), 3); i++) {
            String firstPart = str.substring(0, i);
               // 0.0.0.0 is valid 0.1.0.010 is invalid
            if (i > 1 && firstPart.charAt(0) == '0' || Integer.parseInt(firstPart) > 255) break;
            helper(result, str.substring(i), temp.length() == 0 ? firstPart : temp + "." + firstPart, dotsLeft - 1);
        }
    }
}