合法IP檢測

**合法IP檢測：
首先檢測是否有非法字符，
然後檢測點的數量，
然後檢測是否有點在開頭或結尾或有連續的點連在一起以及每個部分的位數及數值是否合法。**

#include <stdio.h>
#include <string.h>

const int maxn = 110;
char buf[maxn], buf2[maxn];

bool isValidChar(const char *str) {
    for (int i = 0; str[i]; ++i)
        if (str[i] != '.' && !(str[i] >= '0' && str[i] <= '9'))
            return false;
    return true;
}

int countDot(char * str) {
    int cnt = 0;
    for (int i = 0; str[i]; ++i)
        if (str[i] == '.') ++cnt;
    return cnt;
}

bool check(const char *str) {
    int digit = 0, id = 0, len;
    for (int i = 0; str[i]; ++i) {
        if (str[i] == '.') {
            buf2[id] = '\0';
            if (*buf2 == '\0') return false;    // two '.' connected or begin with '.'
            else if ((len = strlen(buf2)) > 1 && *buf2 == '0' || len > 3)
                return false;                   // .012.
            else if (digit > 255) return false;
            digit = id = 0;
        } else {
            digit = digit * 10 + str[i] - '0';
            buf2[id++] = str[i];
        }
    }

    return true;
}

int main() {
    while (gets(buf)) {
        int len = strlen(buf);
        buf[len++] = '.';       // Easy to handle
        buf[len] = '\0';        
        if (!isValidChar(buf) || countDot(buf) != 4 || !check(buf))
            puts("NO");
        else puts("YES");
    }
    return 0;
}