Leetcode——468. Validate IP Address

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题目

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.
https://leetcode.com/problems/validate-ip-address/?tab=Description

Solution

这一题是判断一个输入的字符串是否是合法的IPv4或者IPv6，其实思路很简单，最主要的困难就是各种异常情况的判断。

整理一下：
1、这里要写两个函数，一个是判断一个字符串是否是IPv4，一个是判断一个字符串是否是IPv6。
2、判断IPv4的函数
首先遍历该字符串寻找以分隔符’.’，注意，寻找分隔符只能把前三个数字找出来，所以需要借助字符串的结束符’\0’来把最后一个字符找出来。
之后对每次找到的数字，进行合理性判断。
另外，stoi()是把字符串转化为正数，但是对于长度很长的字符串，比如”11111111111111111111111111111”，stoi是会报错的，所以，在转化之前，可以先对分割后的字符串进行长度判断，如果大于等于4，说明不合法。另外，为了排除”..”即两个分隔符连着一块的，需要对分割后的字符串是否为空进行判断。

3、判断IPv6的函数
基本和IPv4的相同，还是需要注意边界条件

class Solution {
public:string validIPAddress(string IP) {if(IsIPv4(IP)) return "IPv4";else if(IsIPv6(IP)) return "IPv6";elsereturn "Neither";}private:bool IsIPv4(string IP){string temp="";int count=0;int count1=0;for(int i=0;i<=IP.length();i++){if(IP[i]!='.'&&IP[i]!='\0'){if(IP[i]<'0'||IP[i]>'9') return false;temp=temp+IP[i];count1++;if(count1>=4)//exclude：“11111111111111111111111111111.0.1.1”，stoi("111111111111111111111") will report wrongreturn false;}else{if(temp!=""&&stoi(temp)<256&&stoi(temp)>=0)//temp!="" in order to exclude "1..1.1.1"{if(temp[0]=='0'&&temp.length()>1) return false;//exclude：“1.00.1.1”count++;if(count==4) return i==IP.length();}elsereturn false;temp=""; count1=0;}}return false;}bool IsIPv6(string IP){string temp="";int count=0;int count1=0;for(int i=0;i<=IP.length();i++){if((IP[i]>='0'&&IP[i]<='9')||(IP[i]>='A'&&IP[i]<='F')||(IP[i]>='a'&&IP[i]<='f')){count1++;if(count1>4) return false;}else if(IP[i]==':'||IP[i]=='\0'){if(count1==0) return false;//In order to exclude"1::1:1:1:1:1:1:1count++;count1=0;if(count==8) return i==IP.length();}elsereturn false;}return false;}};