93. Restore IP Addresses

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

Example:

Input: "25525511135"
Output: ["255.255.11.135", "255.255.111.35"]

难度：medium

题目：给定一字符串仅包含数字，恢复该字符串为所有可能合法的IP地址组合。

思路：递归

Runtime: 2 ms, faster than 91.11% of Java online submissions for Restore IP Addresses.
Memory Usage: 34.8 MB, less than 0.90% of Java online submissions for Restore IP Addresses.

class Solution {
    public List<String> restoreIpAddresses(String s) {
        List<String> result = new ArrayList();
        if (null == s || s.length() < 4 || s.length() > 12) {
            return result;
        }
        
        restoreIpAddresses(s, 0, 0, "", result);
        
        return result;
    }
    
    private void restoreIpAddresses(String s, int i, int cnt, String str, List<String> result) {
        int sLength = s.length();
        if (i >= sLength && cnt >= 4) {
            result.add(str.substring(0, str.length() - 1));
            return;
        }
        
        int i1 = (i + 1) <= sLength ? Integer.parseInt(s.substring(i, i + 1)) : -1;
        if (i1 >= 0 && cnt < 4) {
            restoreIpAddresses(s, i + 1, cnt + 1, str + i1 + ".", result);
        }
        int i2 = (i + 2) <= sLength ? Integer.parseInt(s.substring(i, i + 2)) : -1;
        if (i2 >= 10 && i2 <= 99 && cnt < 4) {
            restoreIpAddresses(s, i + 2, cnt + 1, str + i2 + ".", result);
        }
        
        int i3 = (i + 3) <= sLength ? Integer.parseInt(s.substring(i, i + 3)) : -1;
        if (i3 >= 100 && i3 <= 255 && cnt < 4) {
            restoreIpAddresses(s, i + 3, cnt + 1, str + i3 + ".", result);
        }
    }
}