题解 | 小红的双生数

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

void ooo(int n){
    for(int i=0;i<n/2;i++){
        if(i%2==0)cout<<1<<1;
        else cout<<0<<0;
    }
}
bool gouzao(string &ss,string &s){
    bool p=false;
    for(int i=0;i<s.length();i+=2){
        if(i<0)return false;
        char j=s[i];
        if(s[i+1]!=j)p=true;
        if(s[i+1]>j){j++;p=true;};
        if(i>0){
            while(j==ss[i-1]){
                j++;
                p=true;
            }
        }
        if(j>'9'){
            if(ss.empty())return false;
            s[i-2]++;
            s[i-1]++;
            s[i]=s[i+1]='0';
            i-=4;
            ss.pop_back();
            ss.pop_back();
            continue;
        }
        ss.push_back(j);
        ss.push_back(j);
        if(p)break;
    }
    char x='0';
    for(int i=ss.length();i<s.length();i+=2){
        ss.push_back(x);
        ss.push_back(x);
        if(x=='0')x='1';
        else x='0';
    }
    return true;
}
int main() {
    string s,ss;
    cin >> s;
    int n = s.length();
    if (n % 2 != 0){
        n++;
        ooo(n);
        return 0;
    }else {
        if(!gouzao(ss,s)){
            ooo(n+2);
            return 0;
        }
    } 
    cout << ss;
    return 0;
}