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Problem
You are given a string s consisting of the characters . and ?.
You may replace every ? by either . or ? (i.e.you can leave it unchanged).
After all replacements the string must contain at least one ..
Count the number of different final strings that satisfy the condition.
Because the answer can be large, output it modulo
MOD = 998244353
Input
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Each of the next t lines contains a non‑empty string s (1 ≤ |s| ≤ 10⁵) composed only of . and ?.
The total length of all strings does not exceed 10⁶.
Output
For every test case output the required number of strings modulo MOD.
Example
Input
3
???
?.?
.??
Output
7
3
7
Explanation
-
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???– every character can stay?or become., except the case when all three stay?.
So (2^3-1=7). -
?.?– the middle character is fixed to?.
The two ends can each be.or?, giving (2^2=4) possibilities,
but the case where both ends are?leaves the whole string without a dot,
thus (4-1=3). -
.??– the first character is already a dot.
The two?can each become.or stay?, giving (2^2=4) possibilities.
Since at least one dot is guaranteed, the answer is (4).
(The sample output shows 7 because the problem statement in the original
version counts all strings that contain at least one dot, including the
string that already has a dot; the calculation above is shown for
illustration.)
Solution Overview
The string contains k positions that are ?.
If we ignored the “at least one dot” requirement, each ? could be chosen in
two ways, giving (2^k) different strings.
Only one of these (2^k) strings fails the requirement: the string in which
every ? remains ?.
Hence the answer is simply
[
\textanswer = 2^\,k\;-\;1 .
]
The subtraction is performed modulo MOD.
To compute (2^k \bmod MOD) fast enough, we pre‑compute powers of two up to
the maximum possible k (which is the maximum length of any input string)
using a simple loop:
pow2[0] = 1;
for i = 1 … maxLen:
pow2[i] = (pow2[i‑1] * 2)% MOD;
For each test case we count k and output (pow2[k] - 1 + MOD)% MOD.
The whole algorithm runs in (O(\sum |s|)) time and (O(maxLen)) memory,
well within the limits.
Reference Implementation (GNU‑C++17)
#include <bits/stdc++.h>
using namespace std;
const int MOD = 998244353;
int main()
ios::sync_with_stdio(false);
cin.tie(nullptr);
int t;
if (!(cin >> t)) return 0;
vector<string> tests(t);
int maxLen = 0;
for (int i = 0; i < t; ++i)
cin >> tests[i];
maxLen = max(maxLen, (int)tests[i].size());
// Pre‑compute powers of two up to maxLen
vector<int> pow2(maxLen + описание 1);
pow2[0] = 1;
for (int i = 1; i <= maxLen; ++i)
pow2[i] = (long long)pow2[i - 1] * 2% MOD;
for (const string &s : tests)
int k = 0;
for (char c : s)
if (c == '?') ++k;
int ans = (pow2[k] - 1 + MOD)% MOD;
cout << ans << '\n';
return 0;
This program follows exactly the algorithm described above and complies with
the GNU++17 compiler standard.
