Validate IP Address

现规定两种IP地址，分别为IPV4和IPV6。IPV4由4个10进制数（0~255）组成，只允许有单独的0出现，数字之间用“.”分隔；

IPV6用8个16进制组成，每个数的长度最多不超过4，允许单独的0出现，数字之间用“：”分隔。

Input: "172.16.254.1"Output: "IPv4"Explanation: This is a valid IPv4 address, return "IPv4".

Input: "2001:0db8:85a3:0:0:8A2E:0370:7334"Output: "IPv6"Explanation: This is a valid IPv6 address, return "IPv6".

部分思路：

1.首先确定开头不能为标点；

2.“.”则不能出现“：”；

3.IPV4有4个“.”；

4.IPV4不能出现字母

5.IPV4的数字长度小于4

自己想的思路时间复杂度太高，在discuss找的几个方法的时间复杂度也是太高。

class Solution {private:    bool validIPv4(string &IP) {        int begin = 0, end = 0, cnt = 0;        while (end < IP.size()) {            int leadingZeroCount = 0, num = 0;            while (end < IP.size() && isdigit(IP[end])) {                num = num * 10 + IP[end] - '0';                if (IP[end] == '0' && num == 0) leadingZeroCount++;                if ((leadingZeroCount > 0 && num != 0)                || leadingZeroCount > 1                || end - begin + 1 > 3                || num > 255) return false;                ++end;            }            if (end == begin) return false;            ++cnt;            if (cnt <= 3) {                if (end >= IP.size() || IP[end] != '.') return false;                begin = ++end;            } else {                if (end != IP.size()) return false;            }        }        return cnt == 4;    }        bool validIPv6(string &IP) {        int cnt = 0, begin = 0, end = 0;        while (end < IP.size()) {            while (end < IP.size() && isalnum(IP[end])) {                if ((IP[end] > 'f' && IP[end] <= 'z')                || (IP[end] > 'F' && IP[end] <= 'Z')                || end - begin + 1 > 4) return false;                ++end;            }            if (begin == end) return false;            ++cnt;            if (cnt <= 7) {                if (end >= IP.size() || IP[end] != ':') return false;                begin = ++end;            } else {                if (end != IP.size()) return false;            }        }        return cnt == 8;    }public:    string validIPAddress(string IP) {        if (validIPv4(IP)) return "IPv4";        if (validIPv6(IP)) return "IPv6";        return "Neither";    }};