LeetCode Validate IP Address Write a function to check whether an input string is a valid IPv4 address or IPv6 address or neither. IPv4 addresses are canonically represented in dot-decimal notation, which consists of four decimal numbers, each ranging from 0 to 255, separated by dots (“.”), e.g.,172.16.254.1; Besides, leading zeros in the IPv4 is invalid. For example, the address 172.16.254.01 is invalid. IPv6 addresses are represented as eight groups of four hexadecimal digits, each group representing 16 bits. The groups are separated by colons (“:”). For example, the address 2001:0db8:85a3:0000:0000:8a2e:0370:7334 is a valid one. Also, we could omit some leading zeros among four hexadecimal digits and some low-case characters in the address to upper-case ones, so 2001:db8:85a3:0:0:8A2E:0370:7334 is also a valid IPv6 address(Omit leading zeros and using upper cases). However, we don’t replace a consecutive group of zero value with a single empty group using two consecutive colons (::) to pursue simplicity. For example, 2001:0db8:85a3::8A2E:0370:7334 is an invalid IPv6 address. Besides, extra leading zeros in the IPv6 is also invalid. For example, the address 02001:0db8:85a3:0000:0000:8a2e:0370:7334 is invalid. Note: You may assume there is no extra space or special characters in the input string. Example 1:

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

Example 2:

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

Example 3:

Input: "256.256.256.256"
Output: "Neither"
Explanation: This is neither a IPv4 address nor a IPv6 address.

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给定一个字符串，判断这个字符串是否符合IPv4或者IPv6的格式。 简单的字符串题目。对于IPv4，需要满足：

1. 只包含数字和点号
2. 包含4个part，每个part用点号分隔
3. 每个part不能有前导0，且数值范围是[0,255]

对于IPv6，需要满足：

1. 只包含数字、冒号和a~f或者A~F
2. 包含8个part，每个part用冒号分隔
3. 每个part可以有前导0，但长度不超过4

把这些规则理清楚，用C++ string写代码，如下： [cpp] class Solution { private: bool checkIPv4Part(const string& part) { size_t len = part.size(); if (len < 1 || len > 3)return false; if (len > 1 && part[0] == ‘0’)return false; for (const auto& c : part) { if (!(c >= ‘0’&&c <= ‘9’))return false; } int v = stoi(part); return v >= 0 && v <= 255; } bool isIPv4(string& IP) { IP += "."; int parts = 0; size_t start = 0; while (true) { size_t pos = IP.find(‘.’, start); if (pos == string::npos)break; if (!checkIPv4Part(IP.substr(start, pos – start)))return false; ++parts; start = pos + 1; } return parts == 4; } bool checkIPv6Part(const string& part) { size_t len = part.size(); if (len < 1 || len > 4)return false; for (const auto& c : part) { if (!((c >= ‘0’&&c <= ‘9’) || (c >= ‘a’&&c <= ‘f’) || (c >= ‘A’&&c <= ‘F’)))return false; } return true; } bool isIPv6(string& IP) { IP += ":"; int parts = 0; size_t start = 0; while (true) { size_t pos = IP.find(‘:’, start); if (pos == string::npos)break; if (!checkIPv6Part(IP.substr(start, pos – start)))return false; ++parts; start = pos + 1; } return parts == 8; } public: string validIPAddress(string IP) { if (IP.find(‘.’) != string::npos) return isIPv4(IP) ? "IPv4" : "Neither"; else return isIPv6(IP) ? "IPv6" : "Neither"; } }; [/cpp] 本代码提交AC，用时3MS。]]>