题解 | #合法IP# 输入不保证正确，只能逐项判断

#include <bits/stdc++.h> using namespace::std;

int main() {

string str;

while (getline(cin, str)) {
    vector<string> vec;
    stringstream ss(str);
    string tmp;
    int flag = 1;
    while (getline(ss, tmp, '.')) {
        vec.push_back(tmp);
    }
    if (vec.size() != 4) {
        cout << "NO" << endl;
    } else {
        for (int i = 0; i < str.size();i++) {
            if ((str[i] != '.') && !isdigit(str[i])) {
                flag = 0;
                break;
            }
        }
        for (auto const &v : vec) {
            if (v.empty()) {
                flag = 0;
                break;
            } else if (stoi(v) > 255) {
                flag = 0;
                break;
            } else if ((v.size() > 1) && ('0' == v[0])) {
                flag = 0;
                break;
            }
        }
        if (flag) {
            cout << "YES" << endl;
        } else {
            cout << "NO" << endl;
        }
    }
}

return 0;

}