题解 | #判断两个IP是否属于同一子网#

精简版：

#include<stdio.h>
int validip(int* arr) {
    for (int i = 0; i < 4; i++) {
        if (arr[i] > 255 || arr[i] < 0)
            return 0;
    }
    return 1;
}
int validmask(int* arr) {
    unsigned mask;
    int i;
    if (!validip(arr))return 0;
    for (i = 0, mask = 0; i < 4; i++) {
        mask = mask * 256 + arr[i];
    }
    mask = ~mask;
    while (mask & 1) {
        mask >>= 1;
    }
    return mask?0:1;
}
unsigned tou(int* arr) {
    unsigned u = 0;
    for (int i = 0; i < 4; i++) 
        u = u * 256 + arr[i];
    return u;
}
int main() {
    int mask[4] = { -1,-1,-1,-1 }, ip1[4] = { -1,-1,-1,-1 }, ip2[4] = { -1,-1,-1,-1 };
    char str[20];
    while (scanf("%d.%d.%d.%d", mask,mask+1,mask+2,mask+3) != EOF) {
        scanf("%d.%d.%d.%d", ip1, ip1 + 1, ip1 + 2, ip1 + 3);
        scanf("%d.%d.%d.%d", ip2, ip2 + 1, ip2 + 2, ip2 + 3);
        if (!(validmask(mask) && validip(ip1) && validip(ip2))) 
            printf("1\n");
        else if ((tou(mask) & tou(ip1)) == (tou(mask) & tou(ip2)))
                printf("0\n");
            else
                printf("2\n");
    }
}

鲁棒版(可以接受地址某段没有数字的数据)

#include<stdio.h>
void to255(char* str, int* arr) {
    int i = 0, j = 0, time = 0, seg = 0;
    while (time < 4) {
        j = i;
        while (!(str[i] == '.' || str[i] == 0)) {
            i++;
        };
        time++;
        seg = 0;
        while (j < i) {
            seg = seg * 10 + str[j] - '0';
            j++;
        }
        arr[time - 1] = seg;

        if (time == 4)break;

        i++;

        if (str[i] == '.' || str[i] == 0) {//fatal error
            for (int k = 0; k < 4; k++) {
                arr[k] = -1;
            }
            return;
        }
    }
}
int validip(int* arr) {
    for (int i = 0; i < 4; i++) {
        if (arr[i] > 255 || arr[i] < 0)
            return 0;
    }
    return 1;
}
int validmask(int* arr) {
    unsigned mask;
    int i;

    if (arr[0] == -1 || !validip(arr))return 0;

    for (i = 0, mask = 0; i < 4; i++) {
        mask = mask * 256 + arr[i];
    }

    mask = ~mask;

    while (mask & 1) {
        mask >>= 1;
    }

    if (mask)
        return 0;
    else
        return 1;

}
unsigned tou(int* arr) {
    unsigned u = 0;
    for (int i = 0; i < 4; i++) {
        u = u * 256 + arr[i];
    }
    return u;
}
int main() {
    int mask[4] = { 0,0,0,0 }, ip1[4] = { 0,0,0,0 }, ip2[4] = { 0,0,0,0 };
    char str[20];
    while (scanf("%s", str) != EOF) {
        to255(str, mask);

        scanf("%s", str);
        to255(str, ip1);

        scanf("%s", str);
        to255(str, ip2);

        if (!(validmask(mask) && validip(ip1) && validip(ip2))) {
            printf("1\n");
        }
        else {
            if ((tou(mask) & tou(ip1)) == (tou(mask) & tou(ip2)))
                printf("0\n");
            else
                printf("2\n");
        }
    }
}