leetcode:468. 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.

题意：

判断字符串是否为合法的IP地址，如果是合法的IPv4地址则返回"IPv4"，如果是合法的IPv6地址则返回"IPv6"，否则返回"Neither"。

解题思路：

纯模拟题，主要考察对字符串的处理能力。大致步骤如下：

1.先判断是何种类型类型的IP地址，进入相应的判别函数，如果都不是就直接返回Neither。

2.构建split()函数，根据分割符个数N将字符串分割为N+1份（空字符串也为一份）。

3.若先找到'.'（IPv4分隔符）符号的：

1）先判断IP在'.'分割符下分割后的段数，若不为4则直接返回Neither。

2）判断分割后每段的字符串长度，若为空串或串长大于3，不合法。

3）判断每段字符串是否有前导0，有则不合法。

4）判断字符串中是否有非数字字符，有则不合法。

5）判断字符串的大小，若大于255则也不合法。

6）若各段字符串均无问题，返回"IPv4"。

4.若先找到':'（IPv6分隔符）符号的：

1）先判断IP在':'分割符下分割后的段数，若不为8则直接返回Neither。

2）判断分割后每段的字符串长度，若为空串或串长大于4，不合法。

3）判断字符串中是否有除[0~9a~fA~F]外的字符，有则不合法。

4）若各段字符串均无问题，返回"IPv6"。

实现代码：

class Solution {
public:
    void split(const string& src, const string& delim, vector<string>& results){
        string str = src;
        int st = 0, ind;
        string substr;
        ind = str.find_first_of(delim, st);
        while(ind != string::npos){
            substr = str.substr(st, ind - st);
            results.push_back(substr);
            st = ind + 1;
            if(st == string::npos){
                results.push_back("");
                return;
            }
            ind= str.find_first_of(delim, st);
        }
        substr = str.substr(st, str.length() - st);
        results.push_back(substr);
        
    }
    string checkIPv4(const string& IP){
        vector<string>results;
        split(IP, ".", results);
        if(results.size() != 4)return "Neither";
        for(int i = 0; i < 4; i++){
            string str = results[i];
            if(str.length() == 0 || str.length() > 3)return "Neither";
            if(str.length() > 1 && str[0] == '0')return "Neither";
            int ind = 0;
            while(ind < str.length()){
                if(str[ind] < '0' || str[ind] > '9')return "Neither";
                ind++;
            }
            int dec = atoi(str.c_str());
            if(dec > 255 || dec < 0)return "Neither";
        }
        return "IPv4";
    }
    string checkIPv6(const string& IP){
        vector<string>results;
        split(IP, ":", results);
        //cout << results.size() << endl;
        if(results.size() != 8)return "Neither";
        for(int i = 0; i < 8; i++){
            string str = results[i];
            if(str.length() == 0 || str.length() > 4)return "Neither";
            int ind = 0;
            while(ind < str.length()){
                char c = str[ind];
                if(c >= '0' && c <= '9' || c >= 'A' && c <= 'F' || c >= 'a' && c <= 'f'){
                    ind++;
                    continue;
                }
                return "Neither";
            }
        }
        return "IPv6";
    }
    string validIPAddress(string IP) {
        vector<string>results;
        int ind = IP.find_first_of(".", 0);
        if(ind != string::npos)return checkIPv4(IP);
        ind = IP.find_first_of(":", 0);
        if(ind != string::npos)return checkIPv6(IP);
        return "Neither";
        
    }
};