POJ 2799 IP Networks

network address是前(32-n)隨意 後n位全零

network mask是前(32-n)全一 後n位全零

本題主要利用位移操作，1ULL表示無符號長整型的常數1，這樣寫可防止不必要的溢位，取反後可以作為mask的列舉然後拿mask和mins或者maxs並一下就得到address了。

程式碼：

#include <cstdio>
#include <iostream>
using namespace std;

const int maxn = 70;

int main()
{

    int n;
    while(scanf("%d", &n) == 1)
    {
        unsigned int maxs = 0;
        unsigned int mins = 0-1;
        //printf("%u\n",mins); 
        int a, b, c, d;
        unsigned int e;
        for(int i = 0; i < n; i++)
        {
            scanf("%d.%d.%d.%d", &a, &b, &c, &d);
            e = ((unsigned int)a << 24) + (b << 16) + (c << 8) + d;
            //printf("%u\n",e); 
            if(e < mins)
                mins = e;
            if(e > maxs)
                maxs = e;
        }
        unsigned int mask;
            for (int i = 0; i <= 32; i++) 
            {
                mask = ~((1ULL<<i)-1U);//注意這裡ULL，是為了防止資料溢位 
                //printf("%u\n",mask);
                if ((mins & mask) == (maxs & mask))
                    break;
            }
        unsigned int ans = mins & mask;

        printf("%u.%u.%u.%u\n", ans >> 24, (ans << 8) >> 24, (ans << 16) >> 24, (ans << 24) >> 24);
        printf("%u.%u.%u.%u\n", mask >> 24, (mask << 8) >> 24, (mask << 16) >> 24, (mask << 24) >> 24);

    }

    return 0;
}