题解 | #合法IP#

#include <iostream>
#include <string>
#include <stdlib.h>
using namespace std;

class LegalIP {
  private:
    string s;

  public:
    //构造函数
    LegalIP(const string s);

    //析构函数
    ~LegalIP();

    //判断IP地址合法性
    const bool judge(void)const;

    //判断字符串是否是0-255的无符号十进制数
    const bool judgeNum(const string s)const;

    //输出结果
    void print(void)const;
};

LegalIP::LegalIP(const string s) {
    this->s = s;
    return;
}

LegalIP::~LegalIP() {
}

const bool LegalIP::judge(void) const {
    //以开始、结束和‘.’为分界点，判断子串是否是0-255的无符号整数
    string sub;

    //记录开始和结束的下标
    int b = 0, e = 0;
    for (int i = 0; i < 4; i++) {
        if (i < 3) {
            for (e = b; this->s[e] != '.'; e++)
                if (e >= this->s.size())
                    return false;
        } else
            for (e = b; e < this->s.size(); e++);

        //避免没出现数字
        if (b == e)
            return false;
        sub = this->s.substr(b, e - b);
        if (!this->judgeNum(sub))
            return false;
        b = e + 1;
    }
    return true;
}

const bool LegalIP::judgeNum(const string s) const {
    int a = atoi(s.c_str());
    if ((a < 0) || (a > 255))
        return false;
    if ((s[0] < '1') || (s[0] > '9'))
        if (a != 0)
            return false;

    //避免出现多余数字
    for (char ch : s)
        if (ch == '.')
            return false;
    return true;
}

void LegalIP::print(void) const {
    if (this->judge())
        cout << "YES" << endl;
    else
        cout << "NO" << endl;
    return;
}

int main() {
    string a;
    while (cin >> a) { // 注意 while 处理多个 case
        LegalIP(a).print();
    }
}
// 64 位输出请用 printf("%lld")