阅读程序（2）

6-11题 组合题

#include <iostream>
#include <string>
using namespace std;

const int P = 998244353, N = 1e4 + 10, M = 20;
int n, m;
string s;
int dp[1 << M];

int solve() {
    dp[0] = 1;
    for (int i = 0; i < n; i++) {
        for (int j = (1 << (m - 1)) - 1; j >= 0; j--) {
            int k = (j << 1) | (s[i] - '0');
            if (j != 0 || s[i] == '1')
                dp[k] = (dp[k] + dp[j]) % P;
        }
    }
    int ans = 0;
    for (int i = 0; i < (1 << m); i++) {
        ans = (ans + 1ll * i * dp[i]) % P;
    }
    return ans;
}

int solve2() {
    int ans = 0;
    for (int i = 0; i < (1 << n); i++) {
        int cnt = 0;
        int num = 0;
        for (int j = 0; j < n; j++) {
            if (i & (1 << j)) {
                num = num * 2 + (s[j] - '0');
                cnt++;
            }
        }
        if (cnt <= m) (ans += num) %= P;
    }
    return ans;
}

int main() {
    cin >> n >> m;
    cin >> s;
    if (n <= 20) {
        cout << solve2() << endl;
    }
    cout << solve() << endl;
    return 0;
}

假设输入的s是包含n个字符的01串，函数solve()所实现的算法时间复杂度是O(n*2^m)（）