题解 | 小红走矩阵

#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;
const int MOD = 998244353;

int main() {
    int n, m;
    cin >> n >> m;
    int a = n + m - 2;
    int b = min(n-1, m-1);
    vector<long long> dp(b + 1, 0);
    dp[0] = 1;
    
    // 递推杨辉三角
    for (int i = 1; i <= a; ++i) {
        // 逆序更新，避免覆盖需要的前置值
        for (int j = min(i, b); j > 0; --j) {
            dp[j] = (dp[j] + dp[j-1]) % MOD;
        }
    }
    
    cout << dp[b] << endl;
    return 0;
}

//另类有趣的解法