题解 | #小红的数字删除#

上次的被hack了，看错题了，没想到删了之后全是0的情况。

#include <iostream>
#include <queue>
#include <map>
#include <set>
#include <cmath>
#include <cstring>
#include <algorithm>
#include <iomanip>
#include <stack>
#include <numeric>
#include <ctime>
#include <string>
#include <bitset>
#include <unordered_map>
#include <unordered_set>

using namespace std;
using ll = long long;
const ll N = 1e5 + 5, mod = 1e9 + 7, inf = 0x3f3f3f3f;

void solve() {
    string a;
    cin >> a;

    int ans = 0, sum = 0;
    map<char, int>mp;

    for (int i = 0; i < a.size(); i++) {
        sum += (a[i] - '0');
        if (a[i] == '3' || a[i] == '6' || a[i] == '9')a[i] = '3';
        else a[i] = (a[i] - '0') % 3 + '0';
        if (a[i] == '3' || a[i] == '0')ans++;
        mp[a[i]]++;
        //cout<<a[i];
    }

    if (sum % 3) {
        if (sum % 3 == 1) {
            if (mp['1']) {
                if (mp['1'] == 1) {
                    if(ans==a.size()-1)ans--;//最后不可以为0
                    for (int i = 0; i < a.size(); i++) {
                        if (a[i] == '1') {                          
                            if(i==0)
                            while (i + 1 < a.size() && a[i + 1] == '0') {
                                i++;
                                ans--;
                            }
                            break;
                        }
                    }
                }
                //if()
                cout << (ans == 0 ? 0 : ans + 1) << '\n';
                return ;
            } else {
                cout << 0 << '\n';
                return ;
            }
        } else if (sum % 3 == 2) {
            if (mp['2']) {
                if (mp['2'] == 1) {
                    if(ans==a.size()-1)ans--;//同上
                    for (int i = 0; i < a.size(); i++) {
                        if (a[i] == '2') {
                            if(i==0)
                            while (i + 1 != a.size() && a[i + 1] == '0') {
                                i++;
                                ans--;
                            }
                            break;
                        }
                    }
                }
                cout << (ans == 0 ? 0 : ans + 1) << '\n';
                return ;
            } else {
                cout << 0 << '\n';
                return ;
            }
        }
    } else {
        if (ans == a.size()) {
            cout << ans - 1 << '\n';
            return ;
        } else {
            cout << ans << '\n';
            return ;
        }
    }
}

int main() {
    ios::sync_with_stdio(0), cin.tie(0), cout.tie(0);

    int t = 1;
    cin >> t;

    while (t--) {

        solve();

    }

    return 0;
}