2月23日两题

The First：https://codeforces.com/contest/2069/problem/C
思路：
找出以1和3围起来的2的组合数量。
1223
代码：

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

const int MOD = 998244353;

int main () {
    ios_base::sync_with_stdio(0); cin.tie(0);
    int T; 
    cin >> T;
    while (T--) {
        int n;
        cin >> n;
        int c1 = 0;
        int c2 = 0;
        int ans = 0;
        for (int i = 0; i < n; i++) {
            int x;
            cin >> x;
            if (x == 1) c1++;
            else if (x == 2) c2 = (2LL * c2 + c1) % MOD;
            else ans = (ans + c2) % MOD;
        }
        cout << ans << '\n';
    }
}

Second：https://leetcode.cn/problems/maximum-sum-with-at-most-k-elements/solutions/3086172/cong-da-dao-xiao-tan-xin-pythonjavacgo-b-vjt2/
思路：
取出每行的相较大的值，之后将所有值排序取k个大值，返回这些值的和。
代码：

class Solution {
public:
    long long maxSum(vector<vector<int>>& grid, vector<int>& limits, int k) {
        if (k == 0) {
            return 0;
        }
        vector<int> a;
        for (int i = 0; i < grid.size(); i++) {
            sort(grid[i].begin(), grid[i].end());
            for (int j = grid[i].size() - 1, p = 1; p <= limits[i]; j--, p++) {
                a.push_back(grid[i][j]);
            }
        }
          sort(a.begin(), a.end());
       long long ans = 0;
        for (int i = 0; i < k; i++) {
            ans += a[a.size() - 1 - i];
        }
        return ans;
    }
};