题解 | #合法IP#

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

int main() {
    string s;
    cin >> s;
    bool flag = false;
    int temp = 0;
    int st = -1, ed = -1;
    int count = 0;
    for (int i = 0; i < s.size(); ++i) {
        if((s[i] < '0' || s[i] > ' 9') ||s[i] == '0' && (s[i + 1] != '.' && i < s.size() - 1)){
            flag = false;
            break;
        }
        if (isdigit(s[i])) {
            st = i;
            while (isdigit(s[i])) {
                i++;
            }
            ed = i;
            temp = stoi(s.substr(st, ed - st));
            if (temp > 255 || temp < 0) {
                flag = false;
                break;
            }else{
                flag = true;
                count++;
            }
        }
    }
    if(flag && count == 4){
        cout << "YES";
    }else{
        cout << "NO";
    }
    return 0;
}
// 64 位输出请用 printf("%lld")