题解 | 滑雪

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

int main() {
    int n, m;
    cin >> n >> m;
    vector<vector<int>> a(n, vector<int>(m));
    for (int i = 0; i < n; ++i)
        for (int j = 0; j < m; ++j) cin >> a[i][j];

    vector<vector<int>> memo(n, vector<int>(m, 0));
    const int dx[4] = {-1, 1, 0, 0};
    const int dy[4] = {0, 0, -1, 1};

    function<int(int, int)> dfs = [&](int x, int y) -> int {
        int& res = memo[x][y];
        if (res) return res;              // 记忆化
        res = 1;                          // 至少包含自己
        for (int k = 0; k < 4; ++k) {
            int nx = x + dx[k], ny = y + dy[k];
            if (nx < 0 || nx >= n || ny < 0 || ny >= m) continue;
            if (a[nx][ny] < a[x][y])      // 严格递减
                res = max(res, 1 + dfs(nx, ny));
        }
        return res;
    };

    int ans = 0;
    for (int i = 0; i < n; ++i)
        for (int j = 0; j < m; ++j)
            ans = max(ans, dfs(i, j));

    cout << ans << '\n';
    return 0;
}