题解 | 判断两个IP是否属于同一子网

#include <bitset>
#include <iostream>
#include <sstream>
#include <string>
#include <vector>
using namespace std;

#define SAMPLE

string toBitString(int num)
{
    return bitset<8>(num).to_string();
}
#ifndef SAMPLE
bool isValidMask(vector<int>& maskArr)
{
    bool switchedZero {false};
    for (const auto& num: maskArr)
    {
        for (const auto& ch: toBitString(num))
        {
            if (ch == '0') switchedZero = true;
            else if (switchedZero && (ch == '1')) return false;
        }
    }

    return true;
}
#endif
#ifdef SAMPLE
bool isValidMask(vector<int>& maskArr)
{
    bool switchedZero {false};
    for (const auto& num: maskArr)
    {
        if (switchedZero)
        {
            if (num == 0) continue;
            return false;
        }
        if (!switchedZero && (num == 255)) continue;
        // now 0 < num < 255
        switchedZero = true; 
        /* solution 1 
        unsigned int n{static_cast<unsigned>(num)}; // could not change num, use n instead, use unsigned in case of overflowing (using move left)
        while (n != 0 && (n & 0b1000'0000) != 0) { // find first 0
            n <<= 1;
        }

        // 剩余的位必须全 0
        if ((n & 0b1111'1111) != 0) {
            return false;
        }
        */
        /* solution 2 */ // no possible overflow
        unsigned int mask = 0b1000'0000;
        while (mask != 0 && (num & mask) != 0) { // find first 0 of num
            mask >>= 1;
        }
        // if num = 1110'0000, after iteration, mask is 0001'0000.

        // 剩余的位必须全 0
        if ((num & (mask -1 )) != 0) { // mask - 1 = 0000'1111
            return false;
        }
        
    }

    return true;
}
#endif

bool isValidIp(vector<int>& ipArr, const string& ip)
{
    string num{};
    istringstream iss(ip);

    while (getline(iss, num, '.'))
    {
        int n{stoi(num)};
        if ((n <= 255) && (n >=0))
            ipArr.push_back(n);
        else 
        {
            return false;
        }
    }

    if (ipArr.size() != 4) return false;

    return true;
}

int main() {
    string mask{};
    string ip1{};
    string ip2{};
    
    while (cin >> mask >> ip1 >> ip2)
    {
        vector<int> ipArr1{};
        vector<int> ipArr2{};
        vector<int> maskArr{};

        bool isValid(true);
        isValid = isValidIp(maskArr, mask) &&
                  isValidMask(maskArr) &&
                  isValidIp(ipArr1, ip1) &&
                  isValidIp(ipArr2, ip2);

        if (!isValid)
        {
            cout << 1 << "\n";
            continue;
        }

        bool isSameSubNet{true};
        for (int i{0}; i < 4; ++i)
        {
            if ((maskArr[i] & ip1[i]) == (maskArr[i] & ip2[i]))
                continue;
            else 
            {
                isSameSubNet = false;
                break;
            }
        }

        if (isSameSubNet) cout << 0 << "\n";
        else cout << 2 << "\n";
    }
}
// 64 位输出请用 printf("%lld")

使用3种方法检查mask是否合法。