C. BAString

把字符串翻转之后处理会更加方便

#include <bits/stdc++.h>
#define ll long long
#define ull unsigned long long
#define rep(i,a,b) for(ll i=(a);i<=(b);i++)
#define dec(i,a,b) for(ll i=(a);i>=(b);i--)
#define pll pair<ll,ll>
using namespace std;
ll INF = 0x7f7f7f7f7f7f7f7f;
const int N = 2e5 + 5;
ll mod = 998244353;

ll n, k, x;
string s;

int main() {
#ifdef _DEBUG
    freopen("input.txt", "r", stdin);
    //freopen("output.txt", "w", stdout);
#endif
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    ll T;
    cin >> T;
    while (T--) {
        cin >> n >> k >> x >> s;
        x--;
        reverse(s.begin(), s.end());
        string ans = "";
        ll i = 0;
        while (i < n) {
            if (s[i] == 'a') {
                ans += s[i];
            }
            else {
                ll j = i;
                while (j + 1 < n && s[j + 1] == s[i]) {
                    j++;
                }
                ll cur = (j - i + 1) * k + 1;
                rep(k, 1, x % cur) {
                    ans += 'b';
                }
                x /= cur;
                i = j;
            }
            i++;
        }
        reverse(ans.begin(), ans.end());
        cout << ans << '\n';
    }
    return 0;
}