题解 | #判断两个IP是否属于同一子网#

#include <stdio.h>

int valid(int a) {
    if (a >= 0 && a <= 255) {
        return 1;
    }
    return 0;
}

int validip(int a, int b, int c, int d) {
    if (valid(a) && valid(b) && valid(c) && valid(d) ) {
        return 1;
    }
    return 0;
}

int main() {
    int s1, s2, s3, s4;
    while(scanf("%d.%d.%d.%d", &s1, &s2, &s3, &s4) != EOF) {
        int ip1_1, ip1_2, ip1_3, ip1_4, ip2_1, ip2_2, ip2_3, ip2_4;
        scanf("%d.%d.%d.%d", &ip1_1, &ip1_2, &ip1_3, &ip1_4);
        scanf("%d.%d.%d.%d", &ip2_1, &ip2_2, &ip2_3, &ip2_4);
        if (!validip(ip1_1, ip1_2, ip1_3, ip1_4)) {
            printf("1\n");
            continue;
        }
        else if (!validip(ip2_1, ip2_2, ip2_3, ip2_4)) {
            printf("1\n");
            continue;
        }
        else if (!validip(s1, s2, s3, s4)) {
            printf("1\n");
            continue;
        }
        // 还需要判断是否前缀全都是1
        unsigned int sub = 0;
        sub = (s1 << 24) + (s2 << 16) + (s3 << 8) + s4;
        int flag = 1;
        int notvalid = 0;
        for (int i = 31; i >= 0; i--) {
            // printf("the i is %d and the (sub >> i) & 1 is %d\n", i , (sub >> i) & 1);
            if (!((sub >> i) & 1)) {
                // printf("the %d i is 0\n", i);
                flag = 0;
            }
            if (flag == 0) {

                if ((sub >> i) & 1) {
                    // printf("the %d i is 1\n", i);
                    notvalid = 1;
                }
            }
        }
        if (notvalid) {
            printf("1\n");
            continue;
        }

        // 如果ip地址和子网掩码都合法，我们对ip地址进行与运算
        unsigned int ip1 = (ip1_1 << 24) + (ip1_2 << 16) + (ip1_3 << 8) + ip1_4;
        unsigned int ip2 = (ip2_1 << 24) + (ip2_2 << 16) + (ip2_3 << 8) + ip2_4;
        if((ip1 & sub) == (ip2 & sub)) {
            printf("0\n");
            continue;
        }
        printf("2\n");

    }
}