LeetCode Online Judge 题目C# 练习 - 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)

        public static List<string> ResotreIPAddress(string s)
        {
            if (s.Length < 4 || s.Length > 12)
                return null;

            List<string> ret = new List<string>();

            int n = s.Length - 1;
            int i = 0;
            for (int j = i + 1; j <= n - 2; j++)
            {
                for (int k = j + 1; k <= n - 1; k++)
                {
                    for (int l = k + 1; l <= n; l++)
                    {
                        string a = s.Substring(i, j - i), b = s.Substring(j, k - j), c = s.Substring(k, l - k), d = s.Substring(l);
                        if (RestoreIPAddressIsValid(a, b, c, d))
                            ret.Add(a + "." + b + "." + c + "." + d);
                    }
                }
            }

            return ret;
        }

        public static bool RestoreIPAddressIsValid(string a, string b, string c, string d)
        {
            // check length for each block
            if (!(a.Length < 4 && b.Length < 4 && c.Length < 4 && d.Length < 4))
                return false;
            // check if each block length == 1 or NOT begin with 0
            if (!((a.Length == 1 || a[0] != '0') && (b.Length == 1 || b[0] != '0') && (c.Length == 1 || c[0] != '0') && (d.Length == 1 || d[0] != '0')))
                return false;

            int aVal = int.Parse(a), bVal = int.Parse(b), cVal = int.Parse(c), dVal = int.Parse(d);

            // check if each block's value is valid
            if (aVal > 255 || bVal > 255 || cVal > 255 || dVal > 255)
                return false;

            return true;
        }

代码分析：

　　BF，但是可以过LeetCode 的 Online Judge. 不知道有没有更好的方法。 这一题的跟之后的 Sudoku Solver 思路相似，穷举所有答案，然后用一个isValid方法筛选。